I'll tell you what Sonic DOESN'T need.
It doesn't need a deep storyline or millions of characters.
It doesn't need characters that BURN UP IN THE ATMOSPHERE only to come back in the next game and then get a whole new game which brings me to another point--
Sonic doesn't need to make itself 'hardcore' by adding guns or motorcycles.
One of the things that makes the older Sonic games so much better than the claptrap Sonic team's been pushing out is that it told a story, but with no words, no speaking or anything like that. You progressed through the game and you knew what was going on. The 3D Sonics suffer from a lot of watching-sonic-run-through-loop-de-loops without being able to control him and they also suffer from a lot of bad stage design. The new 2D Sonics aren't much better off; it seems as though Sonic team doesn't remember what made the old ones so great.
Take Sonic Advance for one. It suffered from boring stage design and too many characters that had very few differences from one another. Sonic Advance 2 completly abstracted the concept of the game in an attempt to get back to Sonic's speedy 'roots', but the levels were again poorly designed. They made no sense. Also, in order to get the chaos emeralds, you had to sometimes stop running and go backwards in a stage to get the 7 special rings and THEN make it to the end of the stage. One thing you shouldn't have to do in a 2D Sonic game is BACKTRACK. In the original games, it seemed as though Sonic was trying to accomplish something with all this random running around, but Sonic Advance 2's presentation felt as though Sonic and his crew were just running around these places aimlessly, until they come across Robotnik. Robotnik seems equally as aimless, and the boss battles were banal and boring. Also, too many characters. Sonic Advance 3 attempted to add some innovation into the mix with it's partnership-based gameplay, and it was a pretty neat mechanic. The stage design was a little better, and the game made the characters seem useful.
I'd say that along with Speed, Sonic needs better stage design, fewer useless characters, fewer 'back from the grave' characters, fewer guns, fewer motorcycles, fewer idiotic storylines and more control over Sonic himself.
All you really need is Sonic, Tails and Knuckles, and there's enough playable characters.
Oh, and Big the Cat? Terrible idea. Worst. Character. Ever. I mean, what board meeting ended with Big the Cat's creation and approval? "Oh, I've got a great idea! Let's take this Sonic game we're making and force you to play the polar opposite of a Sonic game before you can fully finish it!" Yep. The polar opposite of running fast is being an obese cat, fishing. He's even slow witted to boot. Damn it, Sonic Team, get with the program!