Everyone who died before The Snap
By undoing The Snap from Infinity War, Hulk brought back everyone who'd been reduced to ash at the end of Infinity War. But everyone who'd died before Thanos snapped, including Avenger frenemy Loki (whose neck Thanos brutally snapped right in front of Thor), Vision (whose Mind Stone Thanos ripped out of his head to complete his gauntlet), and Peter Quill's flame Gamora (whom Thanos chucked over a cliff to get the Soul Stone), stays very much dead.
That doesn't mean that they won't be back in some form. If you've been keeping tabs on the new Disney+ streaming service hitting our laptops later this year, you know that Marvel already has a few shows planned that involve a couple characters who weren't back for Endgame. WandaVision will bring back Vision somehow for what is hopefully a very cozy cooking show starring two of the Avengers' weirdest members. Vision's body, as far as we know, is still in Wakanda, and now that Shuri's been un-Snapped, who knows what she might be able to do with what remains of his brain in there? Or it could be an Infinity War prequel! Who knows! And Loki, well, Loki's the trickster. If he wants to come back, he will. (He also has a Disney+ show in the works, but we have no idea when or where that's set either.)
Endgame did find a very creative way to bring Gamora back, thanks to the powers of time travel and Thanos figuring out the Avengers' plan. When he follows them through the hole in time their machine created, he brings a past version of Gamora -- who, by the way, has no idea who Peter Quill or any of the Guardians of the Galaxy are -- with him to the future, who then escapes, helps all the heroes foil his plan, and then runs off somewhere. Presumably, the third Guardians movie will be all about Thor and the rest of the crew setting out to find her.