Ant-man is a funny Marvel hero who can became a black horse of this upcoming “Avengers: Endgame” movie.
We know that in continuation of “Avengers: Infinity War” our heroes or those which left, will try to figure out how to reverse what Thanos did. He wiped out half of the universe and we believe that it can be reversed. Doctor Strange in his visions about possible ways of beating Thanos saw this one chance.
What relates to Ant-man. From his last solo movie “Ant-Man and the Wasp” we know that he trapped in the Quantum Realm. It was like death, everyone expect that they have never seen him again. Then we see him in “Avengers: Endgame” trailer. He is in good shape and he exists. How it is possible? Probably he on his own or with somebody’s help came back from Quantum Realm. So it is more than likely the Antman escaping from the alternate dimension could train the rest of the Avengers on how they can use it for their benefit.
We don’t know much about this Quantum Realm apart this that time vortexes exist. Just to mention a scene from “Doctor Strange” in which Ancient One ( in this role Tilda Swinton) was able to throw Doctor Strange ( in this role Benedict Cumberbatch) into Quantum Realm thanks to her magic abilities. Additionally also in animation “Spiderman: Into the Spiderverse” we have some device which can connect with other dimensions and take someone from there into current world.
Of course, producers or actors or anyone else have not revealed how this travel through dimensions could be used. But it is quite sure that heroes need to use this possibility to beat Thanos and use Ant-man knowledge. This is also the reason (probably) why he came to Captain America and Black Widow. Also, Kevin Feige confirmed that Antman and Wasp would be an integral part of the Marvel Universe as it moves forward.