If you could hand pick 2 characters to come back (Megatron, Brawl, Bonecrusher, Frenzy, Jazz, Blackout, or Barricade (I am aware he never died, but it appears that he isn't coming back in the sequel)) Just choose two that you think could have been worked back in well. I think that Megatron and Bonecrusher would be the best choices because Megatron just HAS to come back for at least one more movie (I know that he really is coming back). Even if they had him die he would have to come back for #2 then maybe 3 if they wanted. He is too major of a character to ignore. Bonecrusher would be great because he could be the one that summons the Constructicons together and he could be part of Devastator (either replacing someone or becoming part of the head/torso along with Scavenger and Mixmaster. Just my opinion but who would make the most sense coming back from the dead?