Quicksilver

Marvel can’t keep its heroes dead – so the Civil War might bring the resurrection of Quicksilver. Even if Joss Whedon said that the character was definitely dead in the MCU, Avengers: Age of Ultron also revealed the means of resurrecting metahumans, using the same apparatus that healed Hawkeye. Take into account Quicksilver’s own regenerating ability (as seen when the the soldiers shoot him) and we might have an answer.

As seen throughout all of the MCU movies, HYDRA pops up everywhere to gather artifacts. So maybe the organization has also acquired Quicksilver’s body and is now trying to resurrect it. We don’t know how that will work though, since Scarlet Witch has already chosen a side, but the speedster can always be brainwashed.

But we do know that Tony Stark, especially if he is manipulated by Zemo, has every reason to want Quicksilver on his side.