My theory (Spoilers for IW, obv.):
Avengers 4 will be a Tony Stark Revenge plot. After seeing Peter die in front of him, he returns home to find that Pepper has also vanished. This sets him on a course to find Thanos and, more specifically, the time stone so that he can reverse what happened (As evidented by Strange saying that "this is the only way" prior to his death). In Infinity War, when Thanos wanted the Soul stone, he had to sacrifice Gamora. My over-arching theory is that, by the end of Avengers 4, Tony will have to make a similar sacrifice to use the time stone: He will need to reverse time so far back, to the very beginning (Iron Man 1), so that everyone lives. But in doing so, the life he knows, the people he's met, and the bonds hes formed, never come to pass. The price he pays is losing his love (Pepper), his friends (Avengers), and his fortune (his business). This allows for Marvel to reboot / tell alternative storylines within the MCU and not be bogged down by existing contracts or actors who are done with their roles.
The big hole in my theory is that it potentially erases the entire MCU in one fell swoop, and makes all the previous films essentially non-existent. So while not iron clad, I think some form of Tony's sacrifice will require him to not be a part of what he has built.