Thanos and the Reality Stone.
The trailer has not shown much footage for what I believe will be the fight for the Reality Stone.
I think this fight involved the Guardians, teaming up with Thor, Iron Man, Doctor Strange and Spider-Man.
Leaked footage shows the Guardians sneaking into the Collectors museum where Thanos is torturing him to give him the Reality Stone. The Collector claims he sold it because he didn't know what it was but we know this is not true because he explains his knowledges of the Infinity Stones in Guardians of the Galaxy.
My prediction is that Thanos will get the Aether from the Collector in the scene, but then the Guardians will chase him to regain the Stone. There they will be met by some of the Avengers mentioned above and they will fight to take the Reality Stone back.
I do think they will fail to get it back and Thanos will have the Stone in the Gauntlet by the end of the film.