One More Day was the second time Marvel came close to resurrecting Gwen Stacy. As Rich Johnston noted, back when he was writing about comics gossip in his All the Rage bulletin, John Byrne and Howard Mackie had a similar idea.
Earlier this week, John Byrne revealed the Spider-Man creative team's proposed revamping of Spider-Man's world in a Bobby Ewing style. First, the creative team would have put Peter Parker through the worst of it, until he considers ending it all. At that point, he'd find himself on the bridge where Gwen Stacy died, offering his soul if the clock could have been turned back to simpler times. At which point the Shaper Of Worlds does just that, remaking Spider-Man's world to when he was back in High School, but with the current book's supporting cast, taking place in the modern day. Eventually confronting the Shaper, he discovers nothing can be changed, and his memories of the old world slowly fade away. The team decided though that this kind of event would be too "cosmic" for Spider-Man, who has a "street level" tone.On his message board, John Byrne confirmed that this was considered. It's been noted that this pitch had similarities to what eventually happened in One More Day, a similarly "cosmic" storyline. It had many flaws, and the higher‑ups at Marvel were right to reject it. First, it was odd to have an obscure character like the Shaper of Worlds have such a massive role in a major event in the Spider‑Man franchise. And this would be an oft‑reprinted book, so it could easily be the only exposure many fans have to the Shaper. I’m not even sure if any other book with the Shaper of Worlds has been reprinted.
Changing Peter's age so obviously would also have a tremendous and probably negative impact on Marvel continuity, more so than undoing the marriage. It begs all sorts of weirder questions about the relationship between the Spider‑Man books and the rest of the Marvel Universe. How long have characters like Sandman and the Kingpin, who have had notable encounters with other Marvel superheroes, been villains? How long have the Punisher or Cloak and Dagger been active? What’s the new relationship between Spider‑Man and Daredevil or the Human Torch? The plan showed a complete ignorance of some of the things that made Marvel work, namely the shared universe.
Prior to OMD, there were some rumors that Gwen Stacy’s death would magically be undone, notably due to her role in the third Spider‑Man movie, some J Scott Campbell sketches of the character, her raised profile after the controversy over Sins Past and the correct belief that Peter and Mary Jane’s marriage will be retconned away. Joe Quesada later revealed that he had been pushing for the move. With this single change to Spider‑Man’s history, Peter might never have married Mary Jane, even if he and Gwen were to later break up. And Gwen Stacy would remain the woman who could best compete with Mary Jane for Peter Parker’s affection.
Some have asked what the difference would have been between the resurrection of the Osborns and the possible return of Gwen Stacy. The Osborns were supervillains, so resurrections are more acceptable than in the case of a character who has always been an ordinary teenage girl. With the Osborns, all of the stuff we saw in the original issues still happened. There was just other stuff we weren't privy to.
Even if undoing Gwen's death was the only way to undo Sins Past (and it wouldn't even succeed in that), it would be wrong for Marvel to undo a great story with positive consequences which have affected the vast majority of the Spider‑Man comics of the last three and a half decades in order to undo a six part story and its mediocre four part sequel.
It's also wrong for the magical retcon to undo an actual death at Peter’s request, as this makes him look like a dick for not asking the Shaper of Worlds to undo other events objectively worse than the deaths of loved ones like Gwen (IE‑ any event where more than one person died) and it would beg the question of why he wouldn’t choose to undo the death of Uncle Ben (which would probably have led to Peter going to another college than ESU and never meeting Gwen or Harry Osborn), the deaths of his parents, or Dark Phoenix destroying an entire planet.
Proponents of bringing Gwen back have compared it to Ed Brubaker successfully resurrecting Bucky in Captain America. Personally, I think "The Night Gwen Stacy Died" is on a higher level than Avengers #4, in terms of its critical reputation. Bucky's death was also covered rather quickly, in a three page flashback before Captain America gets used to the modern era and helps the Avengers fight an alien. And there was never a body.
There would be slight advantages to Gwen’s return, and it would certainly muddy the waters about whether or not Peter will get back together with MJ. But it wouldn’t be worth it.
Resurrecting Gwen wouldn't bring about a long term boost in sales, and the regular public knows MJ much better than Gwen. At the same time, Gwen’s death was ultimately a positive thing for the Spider‑Man books. It was a great story, which had a significant and meaningful change to the status quo, and ultimately led to more stories than we would have gotten had Peter and Gwen Stacy just broken up.
The same wouldn’t be true of killing Aunt May or Mary Jane. Gwen’s death was so significant because it happened first. It proved that in the Marvel Universe, anything can happen. Bringing her back would be too much of a sign that nothing is permanent.
The question of whether Gwen's death should be erased was the most prominent difference between JMS's plans for One More Day, and what Marvel published.