Here's the dilemma:
Sonic Team can make a good Sonic game. Low speed, 2D, exploration rewarded by Special Stage rings, Super forms in every stage, etc. The works.
The problem is that implementing any one of those things goes against textbook marketing, which tells you to be NEW AND EXCITING OMFG without EVER basing your product on an older version or older product.
However, as we can see with the recent slew of Sonic games and 1P Mario Kart Wii even Kingdom Hearts 2, then as positive examples, Mega Man 9 and Super Smash Bros. Brawl, that generic marketing concept does not apply to nor does it mesh well with Videogame Design. You must use tried-and-true formulas in game design. You must stick with what works. Deviate the slightest the wrong way, and it'll shatter the whole **** project.
You can, however, add small changes one at a time like Sonic Advance 3 had the Partners, or how Rush introduced the Rush Gauge. Don't let them pile up, though, and definitely don't let bad ones stay (like rail-grinding).