Sonic's overall representation is fine, it's just got some questionable choices: Namely having a Lost World stage of all things to be the compliment to Green Hill Zone.
Music is...Fine. Disappointingly low on remixes but I'd probably chalk that up to what
Cutie Gwen
said: There was definitely some favoritism when it came to what franchises got remixes. (Looking at you, Mega Man and Castlevania, with your absurd number of songs!) This is a problem that plagues many franchises, though, not just Sonic.
Wario has, like, so many good tracks that just get ignored but whatever.
Sonic's moveset has never been an issue to me. It reflects his abilities in the games just fine: I think the problem people take with it is probably more related to him being annoying to fight on quickplay lag, and yeah, I get that. However, that's not really a fault of the moveset, it's more of a fault of the online being so poorly made that it gives certain character archetypes like Sonic a perceived advantage when there's lag.
Spirits are kind of whatever: They're PNGs, and most of the fan favorites are there, so I don't think they're an issue, personally.
I'd love another Sonic character, but not because I think Sonic "needs" more content, just that I like Sonic and would like to see even more.