I would want a more ASW-like Fire Emblem game, in that the roster isn't crazy huge, but each character has their own nuances and quirks in battle. It would be difficult for the initial entry of a fighting game to have 50+ unique playable characters, that usually comes with age.
With this in mind, I'd put around 20 as a pretty reasonable roster size, possibly consisting of:
Marth
Caeda
Roy
Lyn
Seliph
Leif
Ike
Micaiah
Eirika
Ephraim
Alm
Celica
Robin (With gender swaps)
Corrin (Also with gender swaps)
Chrom
Azura
For some villainous represantation:
Gharnef
Lyon
Black Knight
Gheb Walhart, perhaps?
And the inevitable DLC characters:
Lucina
Tiki
Finn
Maybe a character from #FE, or if not, Hector
There we go, 24 characters in total, not counting any OC's they may decide to add for story purposes.