I'd actually like to see them gathering energon. However since it seems the Transformers can leave Earth at will, it just isn't needed. And if ROTF was an attempt to collect energon, it was a poor attempt.
I would like to see Galvatron and/or Omnicron.
These are popcorn movies. I totally get that. Robots fighting and the occasional shot of Megan Fox. Nuff said. However, there doesn't really seem to be a purpose for the Transformers to remain on Earth. Starscream left and brought back more Decepticons, Megatron after being put back together again went into space with ease. I'd think they would look for other Autobots to destroy since they know where Optimus and his cronies are. Avoid the strongest, take out the weaker and when you've found more Decepticons, come back to earth and wipe out the rest of the Autobots. Cowardly? Sure, but Villians aren't often truly heroic. Just look at Starscream...
Anyway, I'll keep seeing these movies as long as they are made.
Though I though Bay was done with directing them. Guess he came to his senses and couldn't pass up that money...