Outside of the MCU, who could undo the snap?

[Powerboy]

Maybe Q from Star Trek?

That's a good one. I'm not sure whether you would have to be on Q's level or if someone less powerful could do it though. It also depends on how powerful you think Q really is. He could definitely do it by going back to before the Big Bang and changing things or just taking the gems himself. Interesting also that we see no evidence that non-corporeal beings are vulnerable to the snap. The Q as a species may not even care about this.

[The Dog]

As Nik said, that is Giorno.

As for the scale, it's somewhat vague on how far Gold Experience Requiem can affect considering it could do lulzy things like:

spoilers:

Undo the event in which a fired gun's bullet returns to the gun after it had struck someone, thus saving them and later on effectively erasing the concept of death for one man

end of spoilers

And if we take Araki's word, it could possibly ignore the effects of a world event that rebooted the timeline; however, that last one has yet to be proven true in-canon and is more of an easter egg for the various video games so far.

Pertaining to this scenario, could GER undo Thanos snapping his fingers? Yeah. Could it undo the results? That's a bit harder to tell.

spoilers:

GER didn't erase the concept of death for Diavolo. Guy still died, he just came back to life... to die in a different, horrible way, ad infinitum in a 'pocket' reality for such a thing (at least I think it was in a pocket reality). Last we see of the guy is he's curled in a field, screaming at a little girl to leave him alone after suffering a number of cruel fates (what we see are him getting stabbed by a hobo, waking up in the morgue during his own necropsy, and getting startled into the road to be hit by a car). Basically GER subjects the victim to Darkseid's Omega Sanction.

end of spoilers

But could it undo the effects of The Snap? Maybe, if it undoes the snap itself. As you said, it's very vague as to what its limits are, considering that it was seen spoilers:

lol-noping through King Crimson's ability to erase time, even talking to Diavolo during it (GER, not Giorno). GER even states that Giorno has no idea what GER's ability is. And it could (possibly) create pocket realities to force victims to suffer repeated, horrible deaths for all eternity.

end of spoilers

[The Drunkard Kid]

The Super Dragon Balls come to mind.

There was an episode of Rescue Rangers in which Monty ended up with the powers of a genie. By the end of the episode, Chip and Dale end up wishing that none of the events of the episode ever happened. This was more time shenanigans but I imagine it wouldn't be a difficult feat for Genie, since he seemed more powerful.

Given how the snap killed people, I imagine Sailor Moon's silver crystal could work on a smaller scale. Healing the planet wouldn't be an issue, but it's safe to say that it wouldn't work on a universal level.

Sailor Moon has (accidentally) used the crystal to pretty much erase the events of the first season from the timeline, IIRC, so she could theoretically prevent the Infinity Stones from being collected that way.

[Yun Lao]

spoilers:

GER didn't erase the concept of death for Diavolo. Guy still died, he just came back to life... to die in a different, horrible way, ad infinitum in a 'pocket' reality for such a thing (at least I think it was in a pocket reality). Last we see of the guy is he's curled in a field, screaming at a little girl to leave him alone after suffering a number of cruel fates (what we see are him getting stabbed by a hobo, waking up in the morgue during his own necropsy, and getting startled into the road to be hit by a car). Basically GER subjects the victim to Darkseid's Omega Sanction.

end of spoilers

But could it undo the effects of The Snap? Maybe, if it undoes the snap itself. As you said, it's very vague as to what its limits are, considering that it was seen spoilers:

lol-noping through King Crimson's ability to erase time, even talking to Diavolo during it (GER, not Giorno). GER even states that Giorno has no idea what GER's ability is. And it could (possibly) create pocket realities to force victims to suffer repeated, horrible deaths for all eternity.

end of spoilers

I can't recall who mentioned this, but the way I understood it, GER and King Crimson's abilities are different sides of the same coin:

spoilers:

From what I can remember, it was theorized that they each manipulate an aspect of cause and effect through King Crimson's time erasure and GER's ability to nullify the end result. For example, we can see this in action with how they deal with gunfire. King Crimson avoids being shot by removing the cause of him being shot, that being his positioning. GER on the other hand, removed the results of the gunfire but undoing the very act, with the bullet reversing its course and returning to the barrel of the gun. That is why I said that GER erased the concept of death for Diavolo. He could experience the act of dying, but the end result of death would never come.

end of spoilers

Of course, we could just say that they both "just work".

[Güicho]

?


?

[Aura Blaize]

Sailor Moon has (accidentally) used the crystal to pretty much erase the events of the first season from the timeline, IIRC, so she could theoretically prevent the Infinity Stones from being collected that way.

I was fairly certain that she brought the Senshi back to life and erased everyone's memories instead of straight up time manipulation. At the beginning of R she remembers everything that happened in the first season.

[Noldere]

I think minus (from webcomic minus) might have a more than decent chance at it, actually.

If absolutely nothing else, going to each and every planet in the universe and mass resurrecting everyone killed there by the snap should be an easy enough thing to convince her to do- it's the grandmother of all field trips.

[Guy1]

I'd say The Mystic Tribunal have a decent shot. They did undo a history altering wish from an entire dimension away.