The biggest problem in my eyes isn't Sonic, the franchise, the characters, but instead Sonic Team/The makers. They are so clearly out of it when making Sonic 2006, Shadow, Unleashed, and so forth that they clearly have no idea what makes Sonic a Sonic game anymore.
If they are burned out or just don't care anymore and doing it for the sake of making more games/are told to, then they need to stop. Give it to Dimps as they seemed to have fun making it and came close to being right when making it on their own (give or take).
Give it to someone who knows that Sonic is about stage design, running, and being able to find different paths in a platforming sense with minor collection along the way (Chaos Emeralds).
Don't try to make it an RPG that slows the game to a halt or makes us listen to in asinine store. Don't add new characters just for the sake of gimmicks, especially when you have several hundred already that could do the job as well. Don't make gimmicks thinking that somehow that is what fans want when we complain about the game not being good.
I still believe Sonic games can do well, just as Mario and Contra have. Go back and make a new 16 bit game if you have to remind yourself of the joy, like that of Mega Man 9 or your own Sonic Advance titles.
Just don't say "we know what fans want now, a magical hippo that shoots rainbows so that Sonic can transverse between Light and Dark world" or some such nonsense. Say "we listened to fans, are wish to make the game more like the 16 bit and handheld titles you loved so dearly).
If it has to be a 3D game, for Egg's sake, don't just make a brand new game reusing or remaking the same damn engine Sonic Adventure had. That was good for the day, but that's long ago and much better tools and techniques have been made. Ask Nintendo or Naughty Dog for help if you need it for 3D.
If you can't do anything other then more gimicks, more of a mess. Just end Sonic as anything other then a company mascot you re-release titles of.