Toise's Super Money Pick:
Marth
Ike
Roy
No discussion on Marth because it's totally unnecessary.
Regarding Ike: Yes, it's possible that he could leave on the grounds that, well, anything's possible. I'm still ruling it out. If you care for a more in-depth explanation, query away.
*** Roy ***
Right now, Roy's the most likely 3rd character for Fire Emblem (should the series get one). Since I've decided that most people are completely ******** about FE in general and Roy in particular, this thread is mostly gonna be about him.
Q: But Roy's not popular!
A: Yes, he is. Being in Melee did far more for his popularity than any FE game could do for anyone. Yes, not being in Brawl puts Ike ahead of him, but that's about it. No, being an AT didn't do nearly as much for Lyn as being playable did for Marth/Roy/Ike.
Q: But FE fans hate Roy.
A: Nope. Most of the "hate" comes from being a clone/swordsman/male/low-tier character, so we've got a vocal minority that wants someone new. Problem is, nobody can agree on who would be better. Every fan poll has Roy as the highest-voted candidate from Fire Emblem. And that's from the fanbase that actually knows Fire Emblem history.
Q: Couldn't anyone have been done what Roy did in Melee?
A: Probably, assuming they were designed well.
Q: Wait, so doesn't being "made" by Smash kinda cheapen his popularity?
A: Nope. Popularity is popularity, no matter what reason. Roy's not the only nobody to become a star just because of Smash anyway, so meh.
Q: So why not just introduce someone new if fans will eat that up too?
A: This would be a lot easier had Roy never been introduced or if people didn't really care for him after Melee. It's that general "you never forget your first love" type of thing.
Q: Well, what if another new guy comes along?
A: Then Roy's got some serious competition. Entirely possible, but I personally don't expect it at this point. Intelligent Systems (the Fire Emblem team) has made two remakes in the last few years and has stated that they've been thinking about others beyond FE1 and FE3. Barring that, Narihiro (head of IS) has said he wants to make some kind of Fire Emblem All-Stars game. We're probably due for some FE general news soon either way.
Q: Does any of that mean anything for Roy?
A: The staff mentioned remakes for FE4 (because of the whole marriage system) and FE6 (specifically so that Roy could have a proper international game). If we get Narihiro's all-star game, Roy would probably be headlining alongside Marth and Ike (that's another Super Toise $50 Wager, by the way).
Q: Roy in Melee was an advertisement and nothing more.
A: Unproven and almost definitely false given Sakurai's attitude towards character selection -- which, by the way, is entirely his decision. I'm too lazy to start adding links to Sakurai interviews and the old Melee dojo-equivalent right now, but I'll dig them up if someone asks.
Q: We can't bring back characters who were cut.
A: Cutting characters is new for this series and something Sakurai has never talked about. But if other roster-based series are any indication, you're way off base.
*** Other stuff ***
New Guy: Well, this is a wait and see kinda thing. Can we get one? Definitely. Will they be any good? Eh, probably. Will they be female or use something other than a sword? I doubt it, but it's possible. No, Caeda doesn't count.
Old Characters: They're all equally irrelevant, but if you think you realistically justify them besides "why I like this guy," go for it.
Three Fire Emblem characters: Assuming you believe in all the scrapped Brawl data to mean that Roy and Mewtwo were supposed to be in the game (and you should, because I do), then Sakurai already had no problem with three for FE in time for Brawl. Three in SSB4 isn't quite so ridiculous, assuming other neglected series get some lovin', anyway.