I don't know much of the characters except from FE7, Sacred Stones, Awakening, and Conquest. So I'll just limit most of my knowledge to the ones I have played. I also don't know every character in Heroes so if they are in the game I'm sorry.
FE7: Heath, Geitz, and Farina
FE8: Moulder, Natasha, and Colm (I really freaking hope he doesn't get added.)
Awakening: Laurent, Nah, and that's about it. And maybe that one knight... what... what was his name?
Fates: Pffft. Freaking everyone will probably get added, unless they already are.