I have felt for a while that it will be Tony. I think he will sacrifice himself to save everyone else. I felt like that even before I watched IW and knew he survived. Dr. Strange obviously saw something involving Tony because he gave up his stone way too fast to save him even after swearing to protect it at all cost. I think Tony is the missing piece to undoing the snap and all the "deaths". I really hope I am wrong. I love RDJ and his character. But this version of the MCU started with him and I think it will end with him.