WARNING; DO NOT READ ANYTHING IN BLACK IF YOU DO NOT WANT TO BE SPOILED. YOU'VE BEEN WARNED.
Wow, I've been seeing a lot of solo Fire Emblem character topics here... but if you think about it, the entire FE lineup should be discussed as a whole. Unlike asking for other characters (Whether it be Sonic, Little Mac, DeMille, whatever), the inclusion of one FE character dramatically reduces the chances of other characters as a whole -- because there are only a limited number of slots.
So instead of picking which ONE FE character/lord has a chance of making it in, let's take a look at the lineup Nintendo will choose. Here's my analysis of each (Courtesy of FESS for the pictures, which are just necessary for showing who each lord is):
Marth - Lord of Dragon of Darkness (FE1) and Mystery of the Emblem (FE3).
Marth's chance isn't bad. He is, after all the first lord. Also, he's the only one to be the lord of two games. But since Fire Emblem isn't like Zelda, in that pretty much every other game takes place in a new world, he doesn't have that definitive attachment to the series. Still, he's the first. And he appeared in Melee. Frankly, I think his chances are bound to Sigurd's chances.
Verdict - 85% chance of returning. Higher if Sigurd doesn't make it.
FE Gaiden lords (Alm and Celica) - A resounding no for obvious reasons.
Sigurd - Lord of the Genealogy of the Holy War (FE4), first generation
Sigurd has a very good chance, because of his popularity (and Sakurai wanted to include him before). He could potentially replace Marth, which to me is kinda weird since he seems to have the power of Roy. But I'm thinking if Sigurd's in, Marth would have to go. Hey, they're also both Japan-only! (Which may change with the VC) Remember that! Too many blue-haired characters running around would be confusing. Although Ike is also blue-haired, he has a dramatically different outfit, too
Also, if the castle stages in the trailer are rom Fire Emblem, it has a pretty good probably of being Velthomer or Barhara, two castles from Grandbell in FE4, by a comparison of flags. (Most notably, the flags appear to be a sideways version of the animals that adorn the flags of Jugdral, the continent in FE4/5). If that's the case, Sigurd is 90% in. But since this is not definite yet...
Verdict - 70% Chance of making it. Higher if Marth doesn't make it.
Celice - Sigurd's son and Lord of the Genealogy of the Holy War, second generation
Celice... I doubt it. He didn't get mentioned in Sakurai's polls I believe. But if he does decide to go all clone-happy again, Celice might be Sigurd's clone -- maybe faster and weaker.
Verdict - No chance.
Leaf - Lord of Thracia 776 (FE5)
Poor guy. He did appear in FE4, and starred in FE5. But FE5 kinda failed (bad timing, really), and he isn't really popular either. Practically no chance... unless he was a clone (but they'd probably pick one of the other choices here as a clone. IF they use clones, god forbid).
Verdict - No chance.
Roy - Lord of The Sword of Seals (FE6)
Roy has a low chance in my opinion. He was only an advertisement, really. Eliwood would have a higher chance, but I even doubt it for Eliwood.
Verdict - 40% chance of returning.
Eliwood, Lyn, and Hector - Lords of The Blazing Sword (FE7)
Eliwood has a low chance, because he frankly isn't that popular (maybe he is in the US, but I'll explain that in a bit.) Lyn and Hector have pretty low chances as they had "co-starring" roles as lords. Also, they weren't mentioned in the poll, I think... (IIRC, the poll had a bunch of FE1 characters, Sigurd, and a handful of Path of Radiance characters. And for the record, yes the poll doesn't matter much because of its inclusion of ridiculous characters, BUT the characters there have better chances than characters that aren't. Think of it as a bare minimum list of candidates, in a way.) Just because they were in the first US game doesn't make them nearly as popular in Japan. (Yes, I know Sakurai said he'd try to hold back on Japan-only stuff, but the characters should be at least pretty popular in Japan to begin with). However, Lyn is still reasonably popular. Her sword style is different, and she's female. Hector also gets a small boost because of his axe-lord awesomeness.
Verdicts:
Eliwood: 10% chance
Lyn: 50% chance
Hector: 40% chance
Ephraim and Eirika - Lords of The Sacred Stones (FE8)
Ephraim and Eirika are really screwed. They don't have much popularity at all... Only thing Ephraim has going for him is his unique spear-wielding abilities...
Verdicts:
Ephraim: 40% chance
Eirika: No chance
Ike - Lord of Path of Radiance (FE9)
Ike has a VERY good chance of getting in. Ever since PoR, he's topped Sigurd in popularity. Not only that, he's also very popular in the US. He's also the star of so far the most recent FE (but not for much longer). He's been in the polls, AND he has a unique fighting style. (Aether will definitely be in the game >| ) Also, he's known internationally. Wait, can it get even better? Yes it can.
*SPOILER*
Ike is one of the lords in FE10, making him the only playable lord TWICE, other than Marth. Of course, he's also the most recent, so that just gives him even better chances.
*/SPOILER*
I don't really need to say more.
Verdict: 99.9% Ike WILL be in Brawl.
*HUGE SPOILERS FOLLOW!!!!!!!!!*
Micaiah - Lord of Goddess of Dawn (FE10)
Micaiah is indeed the lord of the new Fire Emblem. However, the fact remains that she is a mage, and that just makes it harder for them to implement her. However, she does have a handful of outfits to choose from :D. But anyways, let me make this clear about the possible FE10 leaders. All of them have a lower chance, simply because uses up a solid Tellius slot. With characters from Jugdral (FE4/5) and Elibe (Roy/Lyn/Hector) competing for representation of other FE worlds, characters from Tellius are gonna have much less priority. Because of that...
Verdict - 60%
Sothe - Protagonist in Goddess of Dawn
Sothe had a good chance. His knives offer something different than swords -- probably a really quick Sheik-like play style. Also, he can kick like a badass in the movies. AND he's been in FE9 as well. However, he has a couple problems. One, he isn't the main lord. Two, knives are frowned upon, according to Sakurai. Sure, Sothe can still kick like a badass, but he isn't Sothe without his knives. Also, he has the Ike-pwning-Tellius factor going against him
Verdict - 40%
Elincia - Queen of Crimea, and central character in Part 2 and part of Part 4
Elincia has a very small chance. Again, she's a sword user, which doesn't exactly help. Her role wasn't the main role in either story, even thoguh she was a very important supporting character. Yes, she does have a lord role a few times in FE10, but that's not enough. Again, she has the Ike-pwning-Tellius factor going against her.
Verdict - 1%
Tibarn - King of Phoenicia, and a lord character in part of Part 4
I would really love to see Tibarn in Brawl. He's definitely the defition of badass... He can fight well in human form, and also transform for a whole aerial moveset. Like Sothe, he appeared in both games as well. However, like Elincia, his role wasn't central to either game. And once again, he has the Ike-pwning-Tellius factor going against him.
Verdict - 20%
Black Knight - NOT A LORD.
The Black Knight isn't a lord. Yes, he appears in FE10. Yes, you can control him. Yes, I think I know who he is. That doesn't mean **** though. The Black Knight will NOT make it in Brawl. He's way too overpowered (can you imagine throwing a huge pile of armor around, or have him jumping around!?), his role wasn't central enough, he was a cliche character, he has the Ike-factor going against him, and again, he's too ridiculously strong. I just put him here so I could say that. His ONLY ONLY ONLY possible chance is because he's an antogonist character. That gives him a 1% chance, with me being generous.
Verdict - 1%
As for all other non-lords: 0%
So overall, Ike definitely has a FE slot. Fire Emblem can have anywhere from 2 to 4 slots, practically speaking. Also, Marth will definitely be guaranteed if 3 or 4 slots are there. So given that, here are the most possible lineups in my opinion (1 being the most likely in each case):
Two slots
1. Ike and Marth
2. Ike and Sigurd
3. Ike and Lyn
Three slots
1. Ike, Marth, Sigurd
2. Ike, Marth, Lyn
3. Ike, Marth, Micaiah
4. Ike, Marth, Roy
5. Ike, Marth, Sothe
Four slots
1. Ike, Marth, Sigurd, Lyn
2. Ike, Marth, Sigurd, Roy
3. Ike, Marth, Sigurd, Micaiah
4. Ike, Marth, Roy, Sigurd
5. Ike, Marth, Sigurd, Sothe
Wow, I've been seeing a lot of solo Fire Emblem character topics here... but if you think about it, the entire FE lineup should be discussed as a whole. Unlike asking for other characters (Whether it be Sonic, Little Mac, DeMille, whatever), the inclusion of one FE character dramatically reduces the chances of other characters as a whole -- because there are only a limited number of slots.
So instead of picking which ONE FE character/lord has a chance of making it in, let's take a look at the lineup Nintendo will choose. Here's my analysis of each (Courtesy of FESS for the pictures, which are just necessary for showing who each lord is):

Marth - Lord of Dragon of Darkness (FE1) and Mystery of the Emblem (FE3).
Marth's chance isn't bad. He is, after all the first lord. Also, he's the only one to be the lord of two games. But since Fire Emblem isn't like Zelda, in that pretty much every other game takes place in a new world, he doesn't have that definitive attachment to the series. Still, he's the first. And he appeared in Melee. Frankly, I think his chances are bound to Sigurd's chances.
Verdict - 85% chance of returning. Higher if Sigurd doesn't make it.
FE Gaiden lords (Alm and Celica) - A resounding no for obvious reasons.

Sigurd - Lord of the Genealogy of the Holy War (FE4), first generation
Sigurd has a very good chance, because of his popularity (and Sakurai wanted to include him before). He could potentially replace Marth, which to me is kinda weird since he seems to have the power of Roy. But I'm thinking if Sigurd's in, Marth would have to go. Hey, they're also both Japan-only! (Which may change with the VC) Remember that! Too many blue-haired characters running around would be confusing. Although Ike is also blue-haired, he has a dramatically different outfit, too

Also, if the castle stages in the trailer are rom Fire Emblem, it has a pretty good probably of being Velthomer or Barhara, two castles from Grandbell in FE4, by a comparison of flags. (Most notably, the flags appear to be a sideways version of the animals that adorn the flags of Jugdral, the continent in FE4/5). If that's the case, Sigurd is 90% in. But since this is not definite yet...
Verdict - 70% Chance of making it. Higher if Marth doesn't make it.
Celice - Sigurd's son and Lord of the Genealogy of the Holy War, second generation
Celice... I doubt it. He didn't get mentioned in Sakurai's polls I believe. But if he does decide to go all clone-happy again, Celice might be Sigurd's clone -- maybe faster and weaker.
Verdict - No chance.

Leaf - Lord of Thracia 776 (FE5)
Poor guy. He did appear in FE4, and starred in FE5. But FE5 kinda failed (bad timing, really), and he isn't really popular either. Practically no chance... unless he was a clone (but they'd probably pick one of the other choices here as a clone. IF they use clones, god forbid).
Verdict - No chance.

Roy - Lord of The Sword of Seals (FE6)
Roy has a low chance in my opinion. He was only an advertisement, really. Eliwood would have a higher chance, but I even doubt it for Eliwood.
Verdict - 40% chance of returning.
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Eliwood, Lyn, and Hector - Lords of The Blazing Sword (FE7)
Eliwood has a low chance, because he frankly isn't that popular (maybe he is in the US, but I'll explain that in a bit.) Lyn and Hector have pretty low chances as they had "co-starring" roles as lords. Also, they weren't mentioned in the poll, I think... (IIRC, the poll had a bunch of FE1 characters, Sigurd, and a handful of Path of Radiance characters. And for the record, yes the poll doesn't matter much because of its inclusion of ridiculous characters, BUT the characters there have better chances than characters that aren't. Think of it as a bare minimum list of candidates, in a way.) Just because they were in the first US game doesn't make them nearly as popular in Japan. (Yes, I know Sakurai said he'd try to hold back on Japan-only stuff, but the characters should be at least pretty popular in Japan to begin with). However, Lyn is still reasonably popular. Her sword style is different, and she's female. Hector also gets a small boost because of his axe-lord awesomeness.
Verdicts:
Eliwood: 10% chance
Lyn: 50% chance
Hector: 40% chance

Ephraim and Eirika - Lords of The Sacred Stones (FE8)
Ephraim and Eirika are really screwed. They don't have much popularity at all... Only thing Ephraim has going for him is his unique spear-wielding abilities...
Verdicts:
Ephraim: 40% chance
Eirika: No chance

Ike - Lord of Path of Radiance (FE9)
Ike has a VERY good chance of getting in. Ever since PoR, he's topped Sigurd in popularity. Not only that, he's also very popular in the US. He's also the star of so far the most recent FE (but not for much longer). He's been in the polls, AND he has a unique fighting style. (Aether will definitely be in the game >| ) Also, he's known internationally. Wait, can it get even better? Yes it can.
*SPOILER*
Ike is one of the lords in FE10, making him the only playable lord TWICE, other than Marth. Of course, he's also the most recent, so that just gives him even better chances.
*/SPOILER*
I don't really need to say more.
Verdict: 99.9% Ike WILL be in Brawl.

*HUGE SPOILERS FOLLOW!!!!!!!!!*
Micaiah - Lord of Goddess of Dawn (FE10)
Micaiah is indeed the lord of the new Fire Emblem. However, the fact remains that she is a mage, and that just makes it harder for them to implement her. However, she does have a handful of outfits to choose from :D. But anyways, let me make this clear about the possible FE10 leaders. All of them have a lower chance, simply because uses up a solid Tellius slot. With characters from Jugdral (FE4/5) and Elibe (Roy/Lyn/Hector) competing for representation of other FE worlds, characters from Tellius are gonna have much less priority. Because of that...
Verdict - 60%
Sothe - Protagonist in Goddess of Dawn
Sothe had a good chance. His knives offer something different than swords -- probably a really quick Sheik-like play style. Also, he can kick like a badass in the movies. AND he's been in FE9 as well. However, he has a couple problems. One, he isn't the main lord. Two, knives are frowned upon, according to Sakurai. Sure, Sothe can still kick like a badass, but he isn't Sothe without his knives. Also, he has the Ike-pwning-Tellius factor going against him
Verdict - 40%
Elincia - Queen of Crimea, and central character in Part 2 and part of Part 4
Elincia has a very small chance. Again, she's a sword user, which doesn't exactly help. Her role wasn't the main role in either story, even thoguh she was a very important supporting character. Yes, she does have a lord role a few times in FE10, but that's not enough. Again, she has the Ike-pwning-Tellius factor going against her.
Verdict - 1%
Tibarn - King of Phoenicia, and a lord character in part of Part 4
I would really love to see Tibarn in Brawl. He's definitely the defition of badass... He can fight well in human form, and also transform for a whole aerial moveset. Like Sothe, he appeared in both games as well. However, like Elincia, his role wasn't central to either game. And once again, he has the Ike-pwning-Tellius factor going against him.
Verdict - 20%
Black Knight - NOT A LORD.
The Black Knight isn't a lord. Yes, he appears in FE10. Yes, you can control him. Yes, I think I know who he is. That doesn't mean **** though. The Black Knight will NOT make it in Brawl. He's way too overpowered (can you imagine throwing a huge pile of armor around, or have him jumping around!?), his role wasn't central enough, he was a cliche character, he has the Ike-factor going against him, and again, he's too ridiculously strong. I just put him here so I could say that. His ONLY ONLY ONLY possible chance is because he's an antogonist character. That gives him a 1% chance, with me being generous.
Verdict - 1%
As for all other non-lords: 0%
So overall, Ike definitely has a FE slot. Fire Emblem can have anywhere from 2 to 4 slots, practically speaking. Also, Marth will definitely be guaranteed if 3 or 4 slots are there. So given that, here are the most possible lineups in my opinion (1 being the most likely in each case):
Two slots
1. Ike and Marth
2. Ike and Sigurd
3. Ike and Lyn
Three slots
1. Ike, Marth, Sigurd
2. Ike, Marth, Lyn
3. Ike, Marth, Micaiah
4. Ike, Marth, Roy
5. Ike, Marth, Sothe
Four slots
1. Ike, Marth, Sigurd, Lyn
2. Ike, Marth, Sigurd, Roy
3. Ike, Marth, Sigurd, Micaiah
4. Ike, Marth, Roy, Sigurd
5. Ike, Marth, Sigurd, Sothe