Those who haven’t watched Ant-Man and The Wasp are probably going to want to sit down and do it before seeing Avengers: Endgame, as it looks like some really big concepts introduced in that movie will play a big role in the new Avengers.
Sometimes, trailers released on other video platforms may contain footage that isn’t meant to be seen due to how the widescreen cropping on YouTube would normally hide it. Last year, an Instagram spot for Avengers: Infinity War spoiled that Thanos did have all but one stones when he arrived in Wakanda, and now an Instagram spot for Avengers: Endgame may have revealed something pretty cool regarding the Quantum Realm.
QuidVacuo on Twitter spotted the shot where Scott is looking at the missing posters, and he is dragging something by a handle:
It looks a lot like the handle for the shrunken Pym Lab from Ant-Man and The Wasp. It’s possible that the miniaturized Quantum Tunnel in Luis’ van is only big enough to send one person into the Quantum Realm, and to send an entire team of Avengers they would need the full size tunnel in the Pym Lab, or other tech it contains, in order to access the Time Vortexes and save the universe.