This would be a good game.
There could be futuristicy levels like "Tech Island" Modern Sonic style.
And there could be levels like Sweet Mountain Moderner Sonic style (3rd person)
In the original time, Eggman is trying to recreate The Time Eater but does something wrong and makes an explosion that sucks him and Sonic (who was about to stop him) into the future. Meanwhile, in the future, Moderner Eggman does the same thing and sucks him and Moderner Sonic into the present.
In the middle of the 2 time portals, both Sonics see each other and both say in their head "Not again", while actually saying "We have to fix this". Then they are sucked through each others portals. Then both Tails' find out and tell the opppposite time's Sonics and Eggmans that they go about like a normal day and what they usually do and eventually they will be back in their own time.
Sonic* Modern/Moderner = *
Tails* (For solving puzzles that open doors)
Moderner Shilver (Accident between Shadow and Silver)
Sonic (w/ friends)
Modern Metal Sonic
Moderner Mech-Egg Knuckles
Moderner Mech-Egg Shadow
Moderner Mech-Egg Silver
Moderner Mech-Egg Blaze
Moderner Mech-Egg Tails
Moderner Mech-Egg Sonic