... sure about seeing Ronan or Malekith again. If you remember, Ronan was blasted to smithereens by Rockets blaster, and Malekith was crushed by his ship on his homeworld. Ultron and Red skull however, is a different story. Ultron was, correct me if Im wrong, was absorbed by the Mind gem on Visions head. And the Red skull was absorbed by the Cube. So this leads me to my conclusion, when Thanos gains all the gems, he could release them from the gauntlet and use them to his own ends. I think it would be cool to see both of them again, especially since such classic characters like Red skull recieved very little screen time.