"Rethink your Heroes". Interesting. For me, that could mean one of three two things:
1. Optimus is truly evil
This might contradict his "forgive me" line in the trailer, but what if Optimus isn't possessed at all? What if he's asking the other Autobots to forgive his actions because he believes he's doing the right thing by bringing Cybertron to Earth? Perhaps something he sees on his journey in space causes him to have this change of heart?
That said, I hope they don't take this route. Fans complained that Prime was too dark in AOE, having him willingly turn evil would be enough for some to pull out their pitchforks and torches. No matter what might happen to him, Optimus would never sacrifice another species for his own.
2. Megatron will be the "hero" of the Last Knight, bringing Optimus out of his trance and helping him save the day
Okay, Megatron probably won't see the error of his ways and become an Autobot or anything, but we do see him talking to what appears to be Lennox in the trailer. Megatron's the only one that stands a chance against Optimus in a fight, and he could see this as his chance to finally put an end to his brother. He an Optimus fight, Optimus breaks free of the Creator's control, and the two team up against their common enemy.
3. I'm overthinking this popcorn flick and it's not going to be the story-driven Transformers movie that I've fooled myself into thinking it's going to be.