Endgame Speculation

There’s no way it would unfold this way, but I like this idea a lot more than the time-travel/alternate-universe theories I’ve read…

Nebula arrives at Earth and learns of the healing particles that Ant-Man brought back from the quantum realm. She decides her sister is more important than revenge and abandons the fight against Thanos, steals the particles (and a ship) and heads off to find Gamora (plot hole: how does she know where to go?).

She heals/resurrects Gamora just as the rest of the Avengers start to fight Thanos.

The Soul stone metaphorically but quite painfully taps Thanos on the shoulder and says something like “Hey, pal, you reneged on your part of the bargain, so I’m going have to undo everything I helped you do.”

People suddenly un-dust, the Soul stone leaves to find a new secret hiding place, in the process destroying the Infinity Gauntlet (quite explosively, scattering infinity stones all over the battlefield), and Thanos is considerably injured (i.e. missing a hand).

Thanos recovers some of the stones, but some are picked up by worthy and/or suicidal Avengers.

Epic battle, some deaths, heroes win, end credits.

Variant 1: Instead of going into hiding, the Soul stone offers its services to Nebula in honor of her sacrifice in giving up revenge on Thanos. Nebula can’t handle the stone by herself (it nearly kills her when she holds it), but Nebula and Gamora together are quite capable of it.

[Edited to add Variant 1 and a couple of minor edits on April 25, 2019, before seeing the movie.]