I defiantly see Marth, Ike, and Chrom.
Marth is like. The face of Fire Emblem arguably. I think his character should change though, not only his design to fit the one in awakening, but I think he should be more of a quick slash character, only a bit slower than Meta Knight. His luck stat is also like superman high, so he should be very agile and fast.
Ike is the powerhouse of the fire emblem reps. Perhaps make him a bit faster, but other than that and some move set changes he's good.
Chrom is also a powerhouse, but he's not nearly as slow as Ike. But for balancing don't make him as strong as he was in his game, perhaps the strength of C. Falcon or a bit better. And he's not amazingly fast, but please god don't make him move like ganondorf, because it seems like that's what they did to Ike.