How Avengers: Infinity War Completely Changes The MCU
4. It Makes The Multiverse Necessary
Doctor Strange mentions that he went into the future and saw 14,000,605 different outcomes of the fight against Thanos, with the Avengers only winning ONE of them.
Obviously, the focus is on the final part of that, but the sheer fact that he sees so many futures is itself quite a big deal, as it would suggest alternate dimensions being in play.
Avengers 4 is going to need to find a way of bringing the dead heroes back, and it would appear that time travel is, for the time being, ruled out, barring Tony Stark or Shuri inventing it. So the other real possibility is that they have to go to alternate dimensions, which itself is a reasonable explanation for where Captain Marvel has been all this time.
This could be a great way of ensuring Ant-Man and the Wasp is a crucial part of Phase 3, as they'll be exploring the Quantum Realm to rescue Janet van Dyne, which will then further open up the possibility of the multiverse. And once that door is open, the possibilities for what they can do increase exponentially, especially when they'll be looking to factor in the X-Men and Fantastic Four at some stage.