I think that it would either be Megatron or Galvatron in TF2. But I think that the story behind Megatron becoming Galvatron might be a bit much, because they'd have to introduce Unicron, which I think is okay but I think that shouldn't give too much away about him. Most likely to keep the story simple and a little more realistic, if Megatron were to turn into Galvatron then he would have to undergo some type of power changing mechanism that would also be weird because he changed his name. Wouldn't make sense unless they follow his true storyline which may also be a bit weird/unrealistic. Therefore Galvatron may be a whole new character altogether, leading the Decepticons against the Autobots because they "killed" Megatron. I've thought of a story where Galvatron helped Optimus and Megatron rule Cybertron, but he left before the planet's civil war on a mission (perhaps to seek Unicron and try to destroy him). Galvatron comes back and finds out his home is destroyed slowly being rebuilt and that the known survivors are onboard the Ark (Autobots) and the Nemesis (Decepticons) correct me if I have that wrong. They are searching for their allies. Galvatron comes into contact with the Nemesis and finds the Decepticons on Earth who are infromed that Megatron was "betrayed" by Optimus. Galvatron then leads the Decepticons. Something along those lines would fit nicely.