Unicron - I REALLY don't see the appeal to this "character". A planet devouring Tf in G1, and a chaos bringer in Prime are tolerable, but anywhere else, I cringe at his mere existence. His gimmick is already overused and now something fresh and new needs to come up.
Megatron - "Sigh", I love Megatron, I really do, but his time is up. What more can they do with him, bring him back to life as Galvatron? Lame for this universe. (especially since I imagined Galvy and Megs as two separate characters here.)
All the dead Bots and Cons
- See above comments for further explanation.
Targetmasters & Headmasters - Again, what is the appeal to these groups.