To me, theres only one option left-Unicron. He shows up with Cyclonus, who goes and brings Megatron's remains to him. Revival into Galvatron, then he goes down to earth and does SOMETHING to make it so Unicron can take over/have lunch. Like eating the Matrix would kill him or something like that, iunno. But otherwise, stick a fork in this continuity, cause i think it may be done.