Sonic 4 was a step in the right direction for me, an understanding of what needed to be done, just with some issues with execution.
Sonic games need to be made without gimmicks. No special "colours", no werehogs, no magic items or guest stars, no big robot walkers for Tails, no weird human stories. No group player-switch gameplay or cylindrical level layout. And greatly reduce the number of co-stars. All you need for a great Sonic game is speed, momentum and fantastic level design. Music and visuals help a lot too.
Some simple improvements to Sonic 4:
Better, slightly less cartoony visuals and a redesign for Sonic that captures sharp, classic coolness instead of modern... smoothness.
Zoom out! We're on HD consoles now, there's no need to match the scale of old.
Physics, whatever engine you were using, burn it to the ground. Put that momentum back in Sonic's shoes.
Better flow and more intelligent level design.
That said I wouldn't mind another good 3D Sonic either. I think SA2 was the last one I could consider good. But those need a big team and heaps of time and investment.