If nothing else, this makes roster prediction easier. Prediction time.
OCs: Shion, Lian, OC Villain (Darios? Yuana?), the other one.
Shadow Dragon: Marth, Caeda, Tiki, Merric, Linde, Draug
Awakening: Chrom, Robin, Lucina, Frederick, Tharja, Lissa/Virion
Fates: Corrin, Ryoma, Xander, Azura, Takumi, Camilla
And then for villains, GGG. Gharnef, Gangrel, Garon.
This is assuming it's taking the Hyrule Warriors route of "small but unique" roster like the sword and game comment implies. Even if other characters are added, I still think a decent roster can be made from these three games alone, in a worst case scenario. That being said it covers basically all bases.
Swords: Marth, Chrom, Lucina, Corrin, Ryoma, Xander, Gangrel, Shion, Lian
Axes: Frederick, Camilla, Garon
Lances: Caeda, Draug, Azura
Bows: (Virion), Takumi
Tomes: Merric (Wind), Linde (Light), Robin (Thunder), Tharja (Dark), Gharnef (Dark)
Staves: (Lissa)
Dragonstones: Tiki, Corrin
Hidden Weapons: None (OC?)
It also fills most if not all Fire Emblem archetypes.
Lords: Marth, Chrom, Lucina, Corrin, Shion, Lian
Paladins: Frederick, Xander
Archers: (Virion), Takumi
Clerics: (Lissa)
Mages: Merric, Linde, Robin, Tharja, Gharnef
Pegasus Knights: Caeda
Wyvern Riders: Camilla
Generals: Draug
Swordmasters: Ryoma
Thieves: Gangrel
Manaketes/Dragons/Transforming: Tiki, Corrin
Dancer/Songstress: Azura
And then the OC villain and the other OC can do whatever isn't filled. The only major thing I think is missing is a Fire Mage (come on, Celica DLC...). The reason I have it as Lissa/Virion is this: if Staves, non-combat weapons, are used, then Lissa. If not, then Virion, since two archers is fine and they push him a lot.
RIP class-based game though.