

Creator's original idea is established years later
Destiny and Mystique being Nightcrawler's biological parents (X-Men, by writer Chris Claremont). Technically not the original intended parentage for Nightcrawler, but one that goes very far back, at least as far back as the early '80s, shortly after Destiny was introduced. It wasn't canonized until 2023.
Roderick Kingsley being the Hobgoblin (Spider-Man, by writer Roger Stern). Fashion designer Roderick Kingsley was originally intended to be the secret identity of the Hobgoblin in 1983. Problem was, Kingsley and the Goblin were seen together in the original story. The editor at the time found the intended explanation that Kingsley had a twin brother decoy to be hokey, so he was instead identified as reporter Ned Leeds. It wouldn't be until 1997 it was established that Leeds was a fall-guy, and Kingsley was the Hobgoblin all along.
Lois Lane learning Clark Kent is Superman (Superman, by Jerry Siegel and Joe Shuster). Lois was originally supposed to learn Clark was Superman very early on, with an unpublished story from 1940 by Superman's creators making the reveal just two years after their debut. It was rejected by editorial for changing the status quo too much. It wouldn't be until 1990 that she actually found out.
When people talk about power creep, characters getting more powerful over time, it seems to usually be in a negative context. Who're some characters you feel have benefitted from it?
This is for a 10+ year old comic so like I said I don't think anyone will care.
I think the Ultimate Peter Parker who supposedly came back to life is actually the Scorpion.
Last we saw of Scorpion he was being taken from the Baxter Building into government custody, way back in 2005's Ultimate clone saga. From there I believe he was handed over to Roxxon's think tank, where they dismantled the exoskeleton grafted to his spine, maybe using their cloning technology to further make him more like Peter (give him the right number of fingers, repair the damage the suit did to his spine). Their goal was ultimately to replace the "asset" they feel they lost when Jessica ran away.
Then, Miles takes down Roxxon and "Peter" is suddenly waking up in an abandoned laboratory. Either a telepath messed with his memories to make him think he's the real Peter or, in my opinion more interesting, maybe he knows he's not the real Peter, but he wants to be so he's faking it.
The explanation that the Oz in his blood simply made him come back never sat right with me, because he woke up in a lab rather than his grave. There's a mystery there, someone did this to him. And I think this is a more compelling answer to that mystery than if he actually was our Peter.