IMO, the perfect roster for the fighting game would be:
P4 MC
Kanji
Chie
Yukiko
Yosuke
P3 MC
Mitsuru
Akihiko
Yukari
Aigis
That way, everyone would have their own unique playstyle due to the weapons they use, and it would have a balanced 5/5 roster between both games. Other characters like Naoto and Junpei might be more popular, but there are already confirmed characters in the game using their weapons (Aigis and P4 MC respectively), so it would be redundant to include them.
That's what I would do, anyway. Maybe ASW doesn't give a shit.