Captain Marvel Will Be the Key to Defeating Thanos
If you haven't heard yet, Captain Marvel is joining the MCU, and she's a pretty big damn deal. We'll see her tale come to life in the movie right before Avengers 4, and the postcredits scene in Infinity War tells us that she'll be vitally important in defeating Thanos.
We actually have a theory about how she can use her Cosmic Awareness to restore reality to a point before Thanos got control of the Infinity Gauntlet. This would mean that all the Avengers who were "killed" can come back and join together to defeat Thanos. For real this time, without someone getting distracted and letting him go.