I haven't seen Age of Ultron, but Ultron is an easy villain to bring back, even if he's seemingly dead in that film. Yellowjacket wasn't necessarily dead, he may've just gotten sucked into the microverse. Someone can pull him out of there, somehow. Ronan is dead, most likely, but who cares? Loki is still floating around. Red Skull can come back. Which meaningful villains have they absolutely killed without wiggle room?
I think any villain killed directly by an infinity stone has the door open to come back. So far, that's Red Skull, Ultron, Malekith, Ronan.