first things first sorry for the Billiam rant
-If you still want a rep from the fourth PS game, then Rika's an easy pick. Very distinct from Alis, represents an important race in the PS lore and is kind of the successor to Nei who's super popular
-I think the Ristar/Vectorman/etc deal kinda just comes down to a difference in perspective. IMO they're kinda small potatoes compared to other Mega Drive games I'd consider to be much more classic (Streets of Rage, Phantasy Star, Shining Force, the Treasure games, et al), much less the whole of SEGA's library, but I get it. For the record I'd probably put at least some Ristar content in if I did my own take, just maybe not a playable spot. By the way, I think Ecco can work, he has a lot of powers in his games that could make him an interesting fighter and you can just make him, like, float
-I think if you're gonna keep the amount of guests, definitely spread the love a bit more across consoles and try to look into franchises that SEGA have affiliated with before or made games for. Miku's a great pick, definitely the one I'd keep if I added just one guest.
-Shining has some pretty interesting characters beyond just the swordsmen. The classic games have centaurs, a robot, a rat person, a dark sorcerer lady, etc. The modern games have a thief catgirl, an elf magician, a literal guitar wizard, etc
-I think 1 for Shining is fine. By the way, if you happen to want more unrepresented series in your roster, three characters I think would be great to have are Bruno Delinger from Dynamite Deka, Cenes Crawford from Thunder Force V and a class from the 7th Dragon games