My idea for a new Sonic game, like Seviper over here, involves the Chaotix.
My idea is like a cross between Knuckles Chaotix, Sonic Adventure and Sonic Heroes. The characters are as follows:
Espio the Chameleon: Fastest, Climbs Walls, Invisible
Vector the Crocodile: Really Strong, Relatively Slow, Can Swim for Long Periods of Time.
Charmy the Bee: Flies Relatively Quickly (Allows for rapid air-travel) But Not For Very Long Distances, Mildly Annoying
Knuckles the Echidna: Really Strong Too, Faster than Vector, Glides, Climbs Walls.
Mighty the Armadillo: Strong (Not as strong as Vector or Knuckles), Fast (Not as fast as Espio), Can Take 2 Hits Before Dropping Rings
Ray the Flying Squirrel: Kinda Quick on the Ground, Flies For Long Distances.
Basically the story is that Sonic loses the Chaos Emeralds, and enlists the Chaotix to find them while he holds off Eggman. What Sonic doesn't know is that Eggman sent Metal Sonic after the Chaotix, so he could get the Chaos Emeralds to power his latest creation: The Egg Chaos, which is the final boss.
How the levels work in this game is that you will start in an Adventure Field (Like in SA) and then find the levels around the area. When you find the level, while it loads, it shows you an image and gives you a description of the level. Once loading is complete, it will go to a Character Select (Like in Knuckles Chaotix). You can either select 2 specific characters based on the level description, or you can get the game to choose 2 random characters.
That's my idea. I hope you liked it :P