Honestly, when it comes to Fire Emblem, it really depends on FE Switch.
Marth is obviously the face of the franchise, mainly because of his appearances in Smash, but also because Lucina, one of the most popular characters, actually pretended to be him. That really helped make Marth more important even if he wasn't in Awakening. As such, he's definitely staying.
Marth is not just important because of Awakening. Remember the two remakes of his games for the DS? Shadow Dragon was released internationally, shamefully New Mystery didn't, but it still counts. He's just the face of Fire Emblem, and always was. He's a definite lock, and his influence in the franchise is massive.
Ike is an EXTREMELY popular character, which is ironic since, iirc, his games didn't do that well. I guess that's another case of people learning of Ike through Smash and taking a liking to him, just like how people who weren't Japanese learned and took a liking to Marth and Roy. With that in mind, he's staying.
Ike isn't popular for nothing. He was about Lucario-popularity before Brawl. His games might not have sold extremely much, but that's cause it where the only internationally released Fire Emblem console games. The quality is still amazing till this day. You know for how much they are selling Fire Emblem Path of Radiance nowadays? Go ahead and look it up. If IS wants easy cash money, they should opt to do FE9 and FE10 remakes.
Don't forget Intelligent Systems also really favors Ike, and they got a lot to say about the Fire Emblem roster in Smash. They work closely with Sakurai. Ike is definitely staying. And he damn better be, as he's one of my mains and my favorite ever Fire Emblem character.
Roy is a weird case. In Smash, he's really popular, but in FE, he's one of, if not the worst Lord of the franchise. Not to mention that, even if slightly Luigified, he's still mostly a clone, which already makes him likely to get the cut.
*deep sigh*
You cannot be the worst Lord if your main exclusive weapons is amongst one of the best weapons in the franchise...
Roy is extremely popular because FE6 revived Fire Emblem. At least in Japan. It was planned for a international release, but IS deemed the difficulty too hard so they started going international with FE7. He's basically the Lyn for Japan, and the face of handheld Fire Emblem games.
And why would they give him a more unique moveset only to cut him the next game? That's pretty damn counter productive.
Robin has a very unique gameplay and he's the playable character of the game that began Fire Emblem's transformation from a series stuck in a sales dark age to one of the biggest Nintendo franchises of the present. I think those two things warrant him to stay.
Lucina's case is similar to Roy, except it's inverted; Smash people don't really care much about her since she's a complete Marth clone without the tipper while she's easily one of the most popular characters in the Fire Emblem franchise ever. I could see Sakurai either taking the time to Luigify her or making her a Marth alt, but she might also get cut too. Getting cut is the least likely thing imo, but it's not impossible.
Corrin is from Fates, the best-selling Fire Emblem title (kinda helps that there's three versions of it, but still), and is really unique despite being like most FE characters in Smash; a swordfighter with a counter. I'd say she's very likely and I say she because I bet female Corrin would be default Corrin simply because of popularity.
As for newcomers, it really depends on when FE Switch is released and/or if Echoes was an influence in the roster desicion. Heck, we might not even have newcomers, but that's not a bad thing because the current FE characters are very good choices.
Really I don't wanna think about that potential Fire Emblem Switch. It hasn't been announced yet. As of now, I don't think any character is specifically more likely than another, but I could see Lyn make it due to her popularity, or Alm because he's recent, decently popular, and easy to include.
Alright, who would you personally want, and who would you most likely see make the jump out of;
Princess Daisy, Waluigi, Fawful or Geno?
Personally?
I'd WANT Fawful to happen... Yet this is an enourmous pipedream. Yes he's sort of the main villain of the Mario & Luigi sub franchise, yet those games don't get any recognision in Smash thus far. Which is a shame, cause it's quality is amazing! Anyway, Fawful gives me chuckles, and he has FURY! So that makes a great Smash character.
Second up, I would chose Geno. Not that I'm an enourmous fan of Super Mario RPG, but I do recognise it's vallue as the start of Mario RPGs in general. Plus, Geno would make a rather interesting character on his own. I once sort of supported the idea, might run another try on SMRPG...
Waluigi is next in line. He's at least got a few basic abilities and his body type and humorous character can lead to something... fun? I used to be a great Waluigi detractor, but truth to be told, Waluigi would've hyped me more than 70% of the Smash 4 newcomers. Give him some Mario Tennis based moveset or something...
Daisy is last cause well... She'd be fine as a Peach alt honestly. Don't kill me for it. But I don't think that her double jump from Super Mario Run justifies a new character. Now Daisy in her
SPORTS outfit on the other hand............