Ant-Man in this concept art has his latest costume from Ant-Man and the Wasp. Captain America’s costume here is identical to the chainmail armor we saw in old Captain America: Civil War concept art, in addition to sideburns we’ve never seen before and lack of shield (old concept art would definitely feature his shield). War Machine is in a completely new armor here as well. Black Widow seems to be in her Iron Man 2 bodysuit but somehow has her batons from Infinity War. The Thanos piece has him wielding a sword which we’ve seen previously in Infinity War promo art. Maybe Avengers 4 is where they have him wield it.
Overall, this is a pretty convincing leak. I’d be super surprised if this turns out to be fanart. It is pretty exciting to see this version of the Avengers together. The OGs plus their new friends.