Now If I had the choice to personally pick a small handful of my personal favorite characters that I would like to see in warriors would be:
FE: Radiant Dawn: Ike, Micaiah, Nephenee(Knight class), Black Knight(since there is no real knight class characters to play from)
FE: Fates: Gunter,(possible Fredrick clone with some unique different moves) and Elffie(knight class)
FE: Awakening: Maybe Walhart(I think it would a interesting idea), Gaius(unique moveset), especially after seeing how they made Olivia's awesome moveset.
FE: Sacred Stones: Eirika, and Emphraim