The only hope I had after Age of Ultron was due to a rather insignificant quote about Quicksilver's power. "He's got increased metabolism...." My thought was that if they wanted, they could bring him back noting that he didn't die but just appeared that way until his metabolism could repair the damage done to him. Kind of a lame way to do it but not completely out of the realm of movie magic. I do also like the Harkness idea above.