After putting some thought into it, I'm deciding to dip my toes in as well. Considering we may get characters from Elibe, Magvel, Tellius or the other games in a possible FE-Warriors 2: Electric Boogaloo, I like the idea of more playable characters from SD, Fates and Awakening.
SD: Katarina or Merric (Mages) Draug (Knight) and Camus (Paladin).
Fates: Effie (Knight), Rinkah (Fighter/Savage) and Nyx (Dark Mage).
Awakening: Gaius (Thief), Nowi (Manakete), and Vaike (Fighter).
And as a bonus, Villains and Future Past!
Villains: Gharnef, Validar and Macbeth, plus Darios.
Future Past (Awakening): Sevara and Inigo (Mercenaries), and Gerome (Wyvern Rider)