It would be much more likely for them to bring back the guy who played Quicksilver in Age of Ultron. There's a very easy way to do it too. When they reverse the snap in Avengers 4, that could undo his death. I bet reversing the snap is exactly how mutants will be introduced in the MCU too. When they reverse the snap, reality will be restored, but it won't be 100% the same as before. A handful of things will be different. For example, the mutant gene didn't exist pre-snap, but it will exist post-snap.
Why? He sucked as Quicksilver compared to Evan Peters. ---