I'm starting a Fan Fiction soon about what happens 3 Weeks after the evens in Sonic Generations.
Here's a starting plot/preview for first chapter:

Eggman and Classic Eggman are stuck in the white world looking for a way out, continuing from where Sonic Generations left off, and they come across what appears to be the remains of Time Eater after Sonic and Classic Sonic destroyed it and trapped the 2 Eggmans in the Endless White World. They decide to team up once again and rebuild Time Eater using it's remains. After they finish, they used Time eater to open a portal and escape the White World. The Storry then shifts to Classic Sonic's Time. Classic Sonic and Tails are hanging out in Green Hill Zone 3 weeks after the events in Sonic Generations. Suddenly Time Eater appears out of nowhere with the 2 Eggmans. They start shooting at Classic Sonic. Classic Sonic is dodging the shots Time Eater is throwing at him. Since the events in Sonic Generations, Classic Sonic's learned a few tricks from his Future Modern self. He continues to dodge Time Eaters projectiles with Boost, then Classic Sonic jumps high,and attempts to homing attack Time Eater. Time Eater opens a portal in front of him so Classic Sonic ends up getting sucked into the time portal. The portal leads to the future. Classic Sonic is falling from the sky, and lands right in front of Modern Sonic while he's racing through Splash Hill Zone. Modern Sonic is suprized and asks what is going on. Classic Sonic tells him everything that happened, then another portal appears and sucks both Sonics in, then the portal closes. They end up in the white world again, seeing zones that only Modern Sonic is familier with, Classic Sonic not knowing any of the places like last time when old zones from his time appeared. The Zones were Splash Hill, Aquarium Park, Wave Ocean, Windmill Isle, Green Forest, Red Mountain, and Power Plant. The 2 Sonics know what needs to be done, and they run through yet another time bending Adventure, more epic than the last time.

Cool but it needs A major overhaul of What stages there are.
In Sonic Gens 1.
There Was Green Hill,Chemical plant,Casino nights,Skyward Sancuary,Speed Highway,City Escape,Seaside hill,Crisis City,Roofton Run, Planet Wisp, Mushroom hill, Emerald Coast, Radical Highway, Water place, Tropical Resort.
So in # 2 There will Need to Be Some Good Stages So I Will Think about it and come up with some.

I like it, but imo, Classic Sonic shouldn't use Modern's attacks, that's what makes him different. Maybe he can use the elemental shields from Sonic 3?

Eh....it could be better. I like what you're trying to do, but at this point it seems like you're trying to tell us what you're saying. As if this isn't the actual story, you're just trying to summarize it. If that makes sense.
The spelling could also be better in a few words, but that doesn't matter that much and is kind of just a pet peev of mine.