My theory:
Tony comes back to Earth with Nebula. The team decides to go to Thanos. Tony retreats into his lab because he can't face him again. Cap v. Tony again because Tony knows they're on a hopeless mission, but Cap can't sit around while people are dead. Banner sticks with Tony since he still can't Hulk out.
Cap M and Thor believe they can kill Thanos together. The team of Cap, Cap M, Thor, Rocket, Nebula, War Machine, and Widow get their asses kicked. Thor dies and goes to Valhalla. War Machine most likely dies. Nebula gets imprisoned or disassembled by Thanos.
Scott gets out of the Quantum Realm and goes to Tony's on the West Coast. Shuri will be with them too. So we will have Scott, Tony, Banner, and Shuri working on quantum tech and a way to go through time vortices to get what they need. I'd expect them to do something with the whole "infinity stone syncing with a body" thing since all but the soul and time stones have bonded to DNA. From that, they'll find a way to copy the stones through showing up at the points in time where the stones are near each other. Also allows for some comedy with them showing up and confusing past versions.
Thor will be dead and in Valhalla but find some small piece of plot necessary information that will lead to him being resurrected and contributing to the reversal of the snap.
Widow will be tasked with finding Hawkeye so he can help guard the quantum realm entrance. I also expect there will be a plan to lure Thanos to them to attempt to steal the soul stone from him, which Hawkeye will probably do.
End of movie would be Thanos attacking the Avengers compound and the snap being reversed, leading to the big 'every hero at once' v. Thanos battle and Cap's 'Avengers Assemble' moment.
There is clever editing in that final scene from the trailer, but I'm certain it's not a scene. It's Scott outside of Tony's cut with Tony's return.
