It's clear that's there's someone worse then Steeljaw awaiting the Autobots, even if they don't know it yet. But Optimus seems to know, and we know as of episode 13 or 14 he'll begin training with Micronus Prime. And I remember their being a rumor far back that there will be a toy for... Megatronus. Does him being the villain make sense? It does to me. Optimus training with the Primes, Megatron is reformed (so they can't use him as a villian), and it's the only other possibility. Unicron would be kinda cliche as well. And he's not been used enough.