I didn't like a lot of stuff in Avengers: Endgame, so I came up with my own way to undo the snap with time travel. To wit, I didn't like the MCU Clip Show aspect, and I didn't like the fact that Captain Marvel was too powerful to really use in the story. So, here's my fix:
Infinity War is the same up until the very end of Nick Fury's scene. He pages...someone. Call her Dr. MacGuffin, because I don't know which comic book character would be best for the role that follows.
Endgame still opens with Hawkeye's family disappearing. Then the view zooms out to some omniscient narrator eye that flits across the landscape, witnessing many other people turn to dust, before zooming in on a hospital and through the window. We see many staff and patients turning to dust. And we see a woman wake up from a coma. Her chart reads "Jane Doe" and her date of admission is in the mid-1990's (so that Marvel can still do a "Nick Fury in the 90's" movie).
We go to the spaceship with Iron Man and Gamora. We skip the whole "OMG! Will they live?!?!?!?" thing and just have a short character moment.
They land on earth. They discuss how they found the energy signature of Thanos using the stones again. They fly to that planet. They find Thanos. He's destroyed 5 of the 6 stones. He still has the Time Stone. There's a fight. Not drawn-out, but not as abrupt as Thor decapitating him. Thanos dies. With his dying breath he uses the Time Stone.
The Avengers find themselves in orbit around the planet where Farmer Thanos is hiding out. Again they fight him. Again he dies. Again he uses the Time Stone with his dying breath. This happens 1-2 more times.
The Avengers decide that instead of an all-out fight, Black Widow should interrogate him. They make it seem like their ship has crashed and she's the sole survivor. She lets Thanos "capture" her. She does her interrogation thing. He says that he destroyed 5 of the 6 stones so that nobody can undo his work, but he can't destroy the Time Stone. Dr. Strange sent the stone to the future and then summoned it back before handing it over, and if he destroys the Time Stone it will have no future and his past self won't be able to use it to do the snap.
"So I can't change the future. But I can change your past. My condolences on your loss." Then he kills himself before Black Widow can stop him.
The Avengers have the Time Stone but no idea how to use it. They return to earth. Thor is depressed that he didn't get revenge. Hulk is working on getting his mojo back. Hawkeye is killing bad guys who survived. Black Widow works on helping him have his redemption arc. There is no "Five Years Later." This all takes place over several months.
Meanwhile, Dr. MacGuffin is awake from her coma. She is given her belongings. There's an old pager. When she turns it on she discovers a distress signal from Nick Fury. She has amnesia. She recovers her strength, gets released from the hospital, and wanders aimlessly. She's homeless, but she discovers that she has powers. Eventually she meets the Avengers.
She's drawn to the Time Stone. She realizes she can use it. She takes the Avengers back to the events of Infinity War. They replace their old selves And We All Agree Not To Ask Any Questions. On Titan, Dr. Strange takes the Stone from her and gives her back her memories. We don't know what she remembers, but she understands everything. They fight Thanos. Strange still hands over the stone.
But Thanos lacks the Soul Stone. He doesn't know where it is. Gamora is still alive. Thanos has changed the past. Gamora tells them it's on Vormir. They come up with an elaborate plan to delay his efforts to find it. Somehow this plan involves everyone except Hawkeye, Black Widow, and Dr. MacGuffin. Those 3 will retrieve the Soul Stone and Dr. MacGuffin will use it to transfer Vision's soul to another body so that they can destroy the Mind Stone. (They can only destroy a stone if they have a being whose powers derive from the stone.)
On Vormir, Black Widow and Hawkeye fight, and Hawkeye dies (because it makes more sense that way, given his arc.) They get the stone. They go to earth and transfer Vision's soul to another body. They're about to destroy the Mind Stone when Thanos attacks with every single asset of his from across the entire galaxy. Way bigger fight than the original battle of Wakanda.
Thor, Cap, and Iron Man fight Thanos. Cap is the last one standing. Then Dr. Strange gathers together every super hero. Massive battle. All hope seems lost. Tony Snaps. There's a funeral.
At the end, Dr. MacGuffin announces that there's still one last deed to do...in the past. She says she needs help. Cap promises to accompany her, and she promises to send him back. "What about you?" he asks. She says "I've already returned to this time once. I don't need to return again."
But Cap doesn't come back. He instead shows up aged, explains that after accomplishing their mission in the 1990's (Cap was not in the Nick Fury Nineties movie, but his presence in the 90's explains a mystery from the movie) Dr. MacGuffin sent him to the 1940's instead, at his request. "What about her?" "She's in a hospital bed with a coma and amnesia. It's what had to be."
There. Everyone winds up where they were, and we get the awesome fight, but without the ridiculousness.
" Columbus wasn’t a profile in courage or brilliance despite the odds, he was a dumb motherfucker that got lucky. Oddly, that makes him the perfect talisman for the Trump era."