Much has been argued on the Internet recently regarding the future of the Marvel Cinematic Universe. Come the culmination of Phase 3 with Avengers: Infinity War in 2019, the next phase will be an interesting challenge for the head honchos at Marvel Studios. No one has any idea what producer-extraordinaire Kevin Feige has planned - but you can bet dollars to donuts that he'll be intent on topping the quality of Phase 3.
With Age of Ultron making slightly less money than the first Avengers instalment, there may be some fear at Marvel that Infinity War could also under-perform. There is a very distinct chance that it could suffer from audience fatigue: there's already a lot of continuity in place for regular filmgoers to follow and it's only going to get more convoluted.
If that reaches breaking point, we could see a variety of changes happening at the onset of Phase 4.
It'll be a case of examining potentially diminishing box office returns (admittedly unlikely, as far as the upcoming Captain America: Civil War goes - which looks likely to go on to be Marvel Studios' top-grossing non-Avengers film) and checking whether any of the franchise luminaries want to stick around; after all, Robert Downey Jr was only just about kept around to butt heads with Cap next year.
Let's take a look at some of the options confronting Marvel come Phase IV; be it continuing their story-lines with current supporting characters, recasting all their major heroes or starting entirely from scratch.