I'm really hoping that, if Snake does get left out, that his moveset gets slapped onto an Advance Wars character. I really like Snake as a character, but his moveset alone is unique and definitely worth salvaging.
Also, I tried out PSAS for the first time this week... There are some really cool things about it, but for the most part it's pretty unspectacular. A lot of the unique things they tried out don't really seem designed around balance or fun.
20 characters is totally fine for the first iteration of a fighting game, though. I mean, Smash 64 had 12. After a full night of trying to play as many characters as I could, I still had quite a few left.