Marth is the frontrunner, definately. Tied with Ike for the only one of the FE lords to have more than 1 main role, popular, and a Smash veteran. Plus, there's going to be a DS remake of the first (I believe) FE, starring Marth. So he won't be "in the past" any more. Far as I'm concerned, he's pretty much confirmed. I can't see him not being there.
After that, I'd say one of the FE10 lords. Most likely the main Lord of RD (Miciah? I still haven't learned her name >.>). Being recent is a plus, and a female magic user would be something different from the normal FE rep. Then again, Sothe appeared in FE9 and FE10, so it could be him as well, even if his role in FE9 was minor.
Just a bit behind FE10 would probably be Sigurd, since he's still the favorite. At least thats what everyone says. Only problem with him is that popularity is all he has. No standout characteristics, and only a single appearence that was a while back.