Whatever story might be written to "undo" the death of Gwen Stacy, the original story will remain. The moment when Cap awakens inside the Avengers' submarine and calls out to Bucky has been severely compromised by the Winter Soldier storyline, but it nevertheless remains a crucial, pivotal moment in Marvel history in a way that no single moment in all of Brubaker's folderol does.
The death of Gwen Stacy was a clear, direct sequence of events that built to that moment with an admirable simplicity and deepening sense of horror. Whatever Shaper-of-Worlds, Cosmic Cube, Watcher-assisted, Mephisto-Knits-an-Infinity-Mitten nonsense is contrived to "undo" that chain of events, it will be a convoluted, ultimately asinine cop-out and cheat.
Well within the bounds of what the super-hero genre does on a routine basis, yes, but over-reliance upon those conventions are what makes the genre more or less completely irrelevant as a body of literature. The good stories, the best of what comics have done, do not rely on that claptrap.
Besides which, it will mean nothing in the long run. Conway's story (not Lee's) still "happened." There will always be alternate realities where Gwen is still dead (as well as those Claremont-inspired paradises where she's a kiss-ass, take-no-nonsense, warrior babe taking on the Multiverse with a sassy smirk, a SHIELD proto-cannon, and a pink, bubble-gum bubble at the ready.) The event itself will be hard-wired into the story loop contrived to undo it, making it in some respects even more a part of Marvel continuity in the sense that without it, the major storyline undertaken to undo it wouldn't have happened.
Spider-Man's marriage to Mary Jane still took place. "One More Day" couldn't have happened if it didn't. And "One More Day" definitely did take place. The marriage still gets referenced and nodded at whenever Peter and MJ feel a sort of weird connection to one another, even though (silly of them) they really don't know each other all that well... Huh. Funny ol' Multiverse...
Being as much a fanboy as anyone, I would love to be handed the keys to the Kingdom (!) and go nuts writing a 22-part crossover series where Peter is given the chance to go back and undo his past transgressions, the price for which is his identity as Spider-Man. (We'll make Ursula the Sea-Witch the major villain in this piece. That Spidey, a poor, unfortunate soul, in pain, in need...)
He gets to operate in his own past as a sort of Ben Reilly, shadowing his younger self. No Spider-powers means no television career. Peter's probably home the night the Burglar breaks in (and he will because Marv Wolfman told us why he was there that night.) Shadow Peter will have to save them all. Can he do it without Spider-Powers? Can he take down Doc Ock? The Vulture? The Green Goblin? All without his spider-senses and amazing agility?
Does he ever come into his own as that hip, swingin' Romita-guy on the motorbike if he isn't Spidey? Would MJ or Gwen give the time of day to the hangdog, embittered kid who never really got the chance to lash out at his foes and face his inner demons? Or does the good work of Aunt May and Uncle Ben bring him out of that angry funk we first found him in? Most importantly in terms of this story, can he save the police captain's daughter from Norman Osborn's vengeance?
Of course, in a Spidey-less world, there would be no reason whatsoever for the Goblin to take Gwen (of all people) to the top the (mumble, mumble) Bridge and knock her off. In a sense, just agreeing to the deal with Ursula should prevent her death. Norman only did it to get back at Spidey and Spidey isn't there anymore, so deal's done, right? Well, nothing's ever that simple for Spidey, is it?
Somehow we'd have to contrive a similar set-up (or just send him back, not to the beginning of his career, but rather, shortly before this story) so that Peter can save her on panel, in as close to a Gil Kane action-packed scene as we can manage...
As much fun as all that would be, in the end, he'd still wind up as Spidey (Holy Mort Weisinger, Batman!) in a present day much the same as he left it, probably for having violated some pre-condition that tricky Sea-Witch slapped onto the whole she-bang, all his hard work over those years, YEARS!, spent in the past, undone in an instant...
Except there's Gwen! Alive again. Somehow. In some way, some part of it all MUST have actually taken place... Even if it was only just that one moment... She's happy, vivacious, beautiful, and maybe just a little sad it never worked out with her and Peter.
He was a great guy, y'know, but what with his friendship with that reckless criminal in the mask, the death of her dad, the hostage thing she went through, it was all just too much... New York was just an awful place for her. And Peter wouldn't come with her when she wanted to leave. His choice, but somehow she's the one who winds up with the consequences for it...
'Cause in Paris, she met up again with this older guy, see? Okay, fine, it was Harry's dad, and together they had...
Cue the next big Crossover as we now have to go back and undo THAT!!! :-)
As much fun as that would be, and as much as I can see Marvel doing a story just like that, the next fanboy or fanboy-oriented writer is going to have to, HAVE TO, come along and put the lie to that one. Not just ignore it. Maybe not undo it per se, but they will have to write something that backs up over it and makes it all neverwas again.
Would that make Gwen dead again? Would that do something else to her? Did the Phoenix Entity meet her halfway down to the water, put the whole thing on pause, discuss philosophy with her, and decide it would be fun to live as a blonde, human college student while Gwen rested up inside a cocoon from "the shock of the fall" having almost killed her? Maybe she froze upon hitting the icy river and the Red Skull was waiting below, ready to train her as an assassin...
Whatever is done to undo my big story, it would be considered criminal NOT to undo it, because, hey, LOOK at all the story possibilities undoing it gives us! Plus, some of the fans hated it, a smaller number loved it, but pissing them off means they'll HAVE to come back and read this new take on it, because they'll have to see if what they loved survived in any way... Most of all, it would bring us back to that wonderful, perfect, Marvel history moment of Gwen's Death...
Which, after all, throughout all of this, never went away.