This means that within the MCU timeline, Infinity War and Avengers 4 will happen one after the other, allowing for the former movie to set up a shocking twist for the latter movie, like a full-blown reality change.
This is obviously in no way official, but for the sake of argument, let’s assume that Ant-Man and the Wasp is also set pre-Avengers: Infinity War.
Josh Brolin did admit that Thanos might not obtain all the Infinity Stones, but that doesn’t mean another individual couldn’t succeed where he failed.
We also can’t discount the possibility that someone else using the Infinity Stones to change reality, as in the Infinity Gauntlet comic book storyline, both Nebula and Adam Warlock briefly possessed those powers.