if by new you mean like, an existing sonic character tht just Wasn't in the advance games? i think big should've been put in!
if it were up to me i'd have put him in stages that're still fast but definitely more puzzley/platforming-oriented, since i tend to headcanon big as the type to think Carefully even if it takes a second rather than just going "oh lol dumb fat guy"
the stages Should still be speedy though. both to keep a gameplay flow goin and also bc i think the idea of big being able to run at cartoonishly fast speeds is rly funny-
but if by new you mean like, a wholly new character? i kinda wish they repurposed/retooled the red panda design they almost used for cream lol . maybe they coulda introduced a character with some kind of prehensile tail like tangle or ears like maxs/"feels" the proto-sonic to use a kinda grabby mechanic? as i'm typing this i'm realizing i'm basically just saying they should've put ristar in. they should've put ristar in.