If Sega were to make a crossover platform fighter, I feel the best choice for a beginner all-rounder/shoto (from a design standpoint, not marketing or in-universe) like Mario to Smash, SpongeBob to Nick, Ryu to Street Fighter, e.t.c. would be AiAi given that all the Sonic characters' unique statistic traits aren't just parts of their character but rather their entire character.
Knuckles could potentially work but he'd have to sacrifice a lot of his power and possibly his digging, as well as downplay his gliding, while Akira would likely be too complex for most beginners. AiAi on the other hand doesn't have any canonical abilties besides "jump into a ball" so you can give him a lot of the standard shoto moves and stats without it seeming too weird (helped by the fact his tail is stubby to the point of almost being non-existant so nobody would complain about no tail attacks) - plus he already is associated with balls so there's an easy shoto projectile.
