I've got it! I know why Optimus called Megatron "Brother". (NO spoilers. Really.)

They're trying to turn Optimus Prime into Hulk Hogan! Seriously, I reckon in the next movie, they'll have him finish whoever the main bad guy is off with the leg drop. And when he's giving orders, it'll be "Autobots, it's time to show the effects of saying your prayers, and taking your vitamins, brothers! Transform and roll out!"
What do you think?
Yeah, that's a no.
What do you think?
Yeah, that's a no.