When is the last time Doom has been used as a VILLAIN? Scott Lang?
He seems used more as an Anti/reluctant hero as of late... not really a true, big bad, steal the cosmic power Doom.
Same with Loki... bad, evil Loki has been gone for awhile until just recently, where the only thing he has done is laugh and troll new Loki.
Sabertooth and Mystique have been overused a bit I would say. Especially since the X-Men have so many interesting villains they could be using.