I'd probably drop the 2D/Classic sections with Mania returning, but I'm not bothered.
A hub world would be fine, but I think they'd need have a reason for it instead of just throwing it in as a "go to a, then b, then c etc". I'm certain I've said this before, but I'd change the controls so boost isn't a button but a reward by keeping the speed up.
Got to say, despite owning a PS4 now, a lot of the "big" titles just aren't grabbing my attention. Think I've just naturally gravitated towards what are considered the mid tier or the indie titles now.