Don't do this to your friend, but yeah.....

Thanos from 2014 timeline follows some of the crew back from their trip back to get stones from past timelines, shows up at the Avengers HQ and wrecks up the place just as Professor Hulk undoes the snap with the Stark Gauntlet. The battle gets grim for Thor, Cap and Tony while the others try to keep the gauntlet from Thanos' army. Cap begins using Mjolnir with his shield and it's awesome but he's still getting his (America's) ass kicked. He stands alone for the moment as Thanos' army and the BO come toward him from Thanos' ship. "On your left!" comes through his earpiece and sling ring portals open around him as Falcon flies in to help. The snapped characters and armies come walking through maybe 15-30 big portals. "AVENGERS...ASSEMBLE!", big awesome fight. Thanos' gets the gauntlet and tries to snap again, but realises too late that Tony has taken the stones onto his hand. Thanos:"I am inevitable". Tony: "I am Iron Man." Tony snaps, dusting Thanos and his army- but Tony dies right afterwards from using the stones. Pepper and Peter Parker hug him with other Avengers around.

Cap goes to return the stones to where they took them from in the past timelines so as not to screw up thos timelines. He stays with Peggy Carter and comes back old, having lived a life with her. Shield to Falcon.