It'd be cool if they made Galvatron appear and fight Megatron in the next film. Without any explanation whatsoever as to why they're two separate characters. A lot of things were never really explained in the movies anyway.
I know the popular theory or belief is that Galvatron turned back into Megatron in TF5. As that's the most logical explanation. But there's sort of never really been any official word on that.
If they run out of cool ideas, just make Galvatron reappear as a separate character and viola instant story!