I loved "Sonic Generations" and consider it to be my favorite game in this franchise since the Sega Genesis days.
As for a future addition to the Sonic franchise, I'd like to play a game where you can play the game from Eggman's perspective and actually work to achieve world/Mobius domination.
Either that or Metal Sonic or the Chaos Emeralds malfunction to where they get so out of control that Sonic and Eggman must team up to put an end to the crisis at hand.
I'll accept any Sonic title idea but I'd be happy to play a sonic game where you can actually have Eggman as a playable character, preferably not a versus/fighter game ala "Injustice: Gods Among Us" but a platformer game or something like that.
Wouldn't it be fun to have Eggman as a playable character? I think it would be very cool to play a Sonic game where you can play as the bad guy and achieve malevolent goals.
On a side note, I know some of you might look at me strange but I consider Dr. Eggman to be the greatest villain/bad guy ever created. No other character has so many songs in his honor or entertained us so much.