Here's an idea... Robotnik launches a missile powered by chaos emeralds at earth and blows it the fuck up. Killing everyone. Leaving Sonic on planet mobius forever and ever... where there are no humans.
I'm sorry, but Sonic Adventure will happen and there's no way to stop it >: And then he was a zombie.
Hmm...But a missle launch would be a nice touch. Like having the final battle against whoever we decide as really a race against time. Something like how the last battle in Sonic 3 (without Knuckles) was while the Death Egg was launching.