Ultron could return, if in fact one of his drones did survive Sokovia.
We never actually saw Hela die, so I'd say she's the best bet of them all.
A lot of people think Red Skull got teleported by the Tesseract, rather than destroyed, so he could return.
But Ronan...I doubt that one. After all, he did get exploded on-screen. That's hard to come back from.