I would take whatever they did with sonic adventure 2 battle (my alltime favorite sonic game) and refine that formula instead of trying to throw out gimicks that really don't work *as in "wherehog" sections, open hub worlds, ect* I would also take out every single character in the whole lore of sonic with the exception of amy, sonic of course, knuckles, dr. robtnik, tails, and shadow. As those characters seem to have the most personality and charm to them.
I do realize that making sonic levels requires a crapload of assets to make a full level. So i suggust instead of making a sonic game for the nexxt generation, make it for Ps2 instead! Many, MANY, people own playstation 2's, and since you are not constrained with making everything with hd graphics *high poly counts, ect* You could build an insane amount of assets MUCH faster leading to MORE sonic levels and MORE fun.
And even if you did want to make it for the ps3 or 360, make it a *remake* of a sonic game that just boosts the resolution to 1080p but not the actual models and make it downloadable off of their online services.
Bottom Line: IF you can make a FUN 3d Sonic Game, not only will people NOT care about the graphics, but they will praise your name from all around the world for finally reviving the series.