Since it uses the full "Sonic the Hedgehog" title a la 06 (and unlike every game since), I'm guessing either a canned 06 DLC episode or (and here's a scary thought) an 06 sequel that got canned after how laughably bad 06 was.
Yeah, Selvaria's unlock is do a super (the ones with the flashy backgrounds) on most matches. It was actually fairly simple once I got the hang on things. Anyway the final stage is Virtual-On for Akira. I just wasn't aware because I think it's from a Dreamcast title I've not played; I've only played the original! I can't take credit for all that SEGA homework anywho; I got tons of help!
If we get another SEGA character, it'll be one of the SEGA Hard Girls; probably Dreamcast with the Dream Blade, given she's already in the game. Even then though, it's not a SEGA crossover fighter, it's a Dengeki one. Stages are nice enough, it's not 'taunting.'