- Transformers have been on Earth for a long time, blah blah blah, stuff we know, medieval stuff. Maybe humans started building and wearing metal armor to honor ancient Transformers?
- Humans know about dragons because of Dinobots and/or possibly Terrorcons. Optimus got f'ed up during his ill-conceived plan to go after the creators, presumably the Quintessons, because Optimus was an idiot for leaving Earth like he did and I do sympathize.
- Unicron is on his way to Earth, and along the way he captures reformats Optimus into something that at least acts like Nemesis Prime.
On a Side Note;
The purple discolouration in Optimus' eyes at the end resembles the transformation of Megatron to Galvatron in the 1986 movie. Megatron was left to drift in space, just like how Optimus is drifting in space in the trailer.
Optimus Prime is now (probably) Nemesis Prime so that's an opposite too. And since normally Primus would be on Cybertron, and since Cybertron was destroyed in TF3, that must mean that Primus is actually on Earth (another opposite). Which would also explain why the Transformers keep coming to Earth, and why Unicron is on his way.
And if Primus is on (in, part of, etc) Earth, then we know that Nemesis Prime will be reformatted again.
The robot in the desert with the sword I think is the new body for Megatron. Not positive though.
Other previously released press stuff:
Hot Rod is also going to appear in this movie, who is traditionally successor to Optimus as leader of the Autobots, and in press release material is said to be Bumblebees brother.
All the Autobots from the last movie are returning. There also might be mini-dinobots?
Also Barricade is somehow back even though he died in the third movie. Finally, the little robot prop the girl was interacting with is named Sqweeks and is blatantly a BB-8 cash in.
Tl;dr We've got knights, nazis, robot dragons, alien robot knights, big effin robot planet's eating other planets, and a whole bunch of exploding stuff. I mean, I'm not going to pretend that these movies are fine works of cinema. I'm not even going to pretend that they are approaching any level of "good". But, what the hell. I'll spend money on it. I'll spend money on it twice. This is his fifth Transformers movie, I know what I'm getting into. I just want Mad Michael to f*** my shit up for two and a half hours.