Disclaimer: I will say what I think they should add to the current series' direction, not to already-out games.
sonic needs a PROPER DESIGNER. And needs to shake off from fans, as all Sega was able to do was to painfully deal a slow return to a verbatim copy of Sonic 2.
They always try to slam something new in, but it is invariably meh and everyone will rage about it. They really need to make a clean game that is not divided in countless random parts to try to not unleash the haters.
They have to make a single sonic that is both original AND good AND "sonic", and I don't think that Sega can pull this off at all.
Down to practical ideas:
-Most important: gameplay at low and high speed should be different and the transition not linear; please get rid of the current half-assed physics-but-not-physics.. that will for sure upset die-hard fans (or haters?) but is OLD.
-at low speed you should be able to control Sonic just like mario, without too much physics kicking in, and long, low and controlled jumps. No floating PLS.
-at high speed Sonic should be basically physics-driven, having puzzles revolving on being able to get there without being hurt and being as stylish as possible. You can bounce from the current surface and brake, curl and uncurl.
-Even if the platforming was slow and awkward, it always was the "meat" of the game, with "speed" being either eyecandy or a reward. Acrobatics and speed management should be part of the game and not eyecandy, eg - acquiring enough speed for a loop is the only way to not die in the lava.
-2D is good for speed, 3D is not.
-Sonic was Canabalt before Canabalt was Canabalt. I urge Sega to notice that.
just leave it to Retro. They are good.
I like that there's currently an "experimental kirby saga" (Epic Yarn, Canvas Course, Mass Attack...) and a "traditional platformer saga". Experimental kirbys are invariably awesome, traditional ones tough feel a little like if not enough creativity pours down to them.
Please make crazier level, crazier mechanics... but not by much
Also, I HATE POWER COMBOS PLEASE DON'T. They kill the pace completely and feel gimmicky.