All you need for a spinoff is for Sonic to not be the main star. For my entry, Sonic and 4 other characters are playable and all 5 stories need to be finished to get to the Last story.
My contest entry is ready. It has been revised a little to add in some new idea I though was cool. Summary: A fusion of the gameplay of Sonic Rush with the gameplay of the Mega Man franchise. (Mostly the combat.) In regards to a visual view of the concept, I've decided to take it a step further. http://media.ds.ign.com/media/794/794195/vids_1.html
Posted here is a link to actual gameplay footage of Mega Man ZX (the latest game in the franchise, coming out 9/12. I pre-ordered it last May) so the judges can get a good picture of what's being fused with Rush gameplay. (Some of the later Mega Man games are capable of moving at a surprisingly fast pace in the hands of someone who knows what they're doing.) The first 2 videos are from a hands on look at an early version of the Japanese version. (Not being played to its max speed potential.) The third video is of a cut scene that is surprisingly good for a DS game. The 4th one is the E3 footage and in the last half, shows just how fast the game is capable of moving at.
-Edit: I have seen images of what the traditional shell might look like. It may look like a souped up SNES controller with twin analog sticks. Any mention of remote based movement for flight are scrapped and replaced with analog movement.