Graviton

In Agents of S.H.I.E.L.D., Franklin Hall – played by Ian Hart – had been using Gravitonium to experiment with gravity, before deeming it too dangerous and trying to destroy it. However, what he didn’t anticipate was a S.H.I.E.L.D. extraction operation disrupting his efforts.

Phil Coulson determined that Hall’s plan would harm much of the local population, but his attempts to stop it resulted in Hall being sucked into it and apparently perishing – meaning Coulson and company think he is dead.

However, his face appeared in the element at the end of the episode in question, suggesting he might still be alive within it.

In spite of the fact that he was a decent guy, he’s going to be pretty annoyed with S.H.I.E.L.D. and, should he re-emerge – which was definitely hinted at – he may well do so as the villainous Graviton. That would be so cool, because he’s a major character in the comic books.