While he is the only villain teased thus far, the consensus among many is that he will be joined by the planet-eater Unicron in the film. The foundation is already laid for that storyline to take place, and if Unicron was included, it might explain how Megatron is returned to his original form.
In the original Transformers movie (the 80's animated one, not the Michael Bay version), a broken Megatron and what remains of his Decepticon forces are wandering through space after a heated battle with the Autobots. Even though Megatron successfully took down Optimus Prime (with a little help from Hot Rod's stupidity), he is also on the brink of death. They end up getting pulled into what appears to be a huge ship, only it speaks.
This is where we first meet Unicron, who enlists Megatron and his cronies to help him destroy the matrix of leadership, the only thing that can hurt him. Megatron insists Prime is dead, but at this point, the matrix has already been passed to Ultra Magnus. He then decks out Megatron and his Decepticons with new armor and weapons and renames Megatron Galvatron.
In the films, Unicron hasn't shown up yet, but in Transformers: Age of Extinction Megatron's consciousness is mistakenly transported into a new body, which is named Galvatron. The image the studio released makes it appear as if he is in his old body at some point, so the question is how does he get there?
The thinking is that if Unicron shows up and follows the same overall plotline, then he would give Megatron a new and improved version of his original form, therefore mimicking the animated movie, just flipped around a bit.
I'll be interested to see which route they take, but I do hope they involve Unicron in some shape or form.