for the sake of arguement, and keeping to the primarily fates shadow dragon and awakening thing for the base game, lets say…
shadow dragon:
Marth
Caeda
Tiki
Minerva
Merric
awakening:
Robin (female and male counting as one just different skins)
Chrom
Frederick
Lissa
Lucina
fates, all three routes:
Corrin (same deal as robin)
Azura
Xander
Ryoma
Shigure
at minimum with the five remaining slots covered by either new characters and or really popular characters from games other then the three featured ones, maybe even a cameo from a dynasty warriors character for fun, thus keeping the playable cast at about 20 in the base game.
ideally though i would say, in the same guidlines again
shadow dragon:
Marth
Caeda
Tiki
Minerva
Merric
Ogma or navarre
Jeorge
Wolf
Midia
Lorenze
awakening:
Robin (female and male counting as one just different skins)
Chrom
Frederick
Lissa
Lucina
Cordelia
Say’ri
Flavia
Basilio
Henry
fates, all three routes:
Corrin (same deal as robin)
Azura
Xander
Ryoma
Camilla
Hinoka
Leo
Takumi
Elise
Sakura
would be more interesting, with the most disputable one being navarre or ogma in my opinion, as either are fine for trying new character interactions, with the main difference being navarres greater popularity or ogma presenting a chance for a more unique moveset. 30 would probably stretch the development time a bit longer and limit the amount of other characters to just new one/the two new lords for the playable cast.
the enemy crew could be divied up like this
shadow dragon:
Gharnef
Michalis
Camus
gras king
awakening:
Validar
Aversa
Walhart
creepy valm mage
fates, all three routes:
Garon
Sumeragi
azuras mother whos name i forget
hans
and whatever other new villains the game wants to use. again this would probably increase the production time but id be willing to wait