The Marvel Universe is vast, and it usually can be divided into specific areas. It gets really gray, however, when creators decide to annex other parts of the lore into their run, and in some cases it completely transplants over. This often happens when they aren't being used in the main part, so they decide to integrate them in their own stories. It's also prominent when the creator worked on them in the past in a different title, so they're essentially revisiting it.
Examples:
- Frank Miller took Kingpin, a lesser Spidey rogue, and made him the big villain to Daredevil. His roles antagonizing Spider-Man after this are patterned after Miller's take on Fisk. Nowadays, Kingpin almost exclusively menaces Daredevil, and whenever he causes trouble for Spidey, it's as a mastermind role rather than a direct combatant (I think after Back in Black no one could take Kingpin against Spidey seriously again).
- Chris Claremont rolled in numerous, often obscure, elements of the MU lore into the X-Men mythos, and are now considered more X-Men than General MU. It's funny considering how "closed-off" from the MU the X-Men supposedly are according to some. This includes Mystique and Deathbird as X-Men villains (originating from Ms. Marvel), Sabretooth as Wolverine's nemesis (originating from Iron Fist), the Captain Britain mythology (originally its own thing), Mojoworld alongside Longshot, Spiral and Mojo (a self-contained miniseries not tied to the X-Men), Adamantium (debuted in Avengers as the metal Ultron as made of), Ka-Zar and the Savage Land (Golden Age), Lady Deathstrike becoming Wolverine's other nemesis (originated from Daredevil), Madame Hydra as a villain the X-Men and New Mutants (originating as a Captain America villain), and Wolverine himself having debuted as a villain in The Incredible Hulk.
- In the Ultimate Marvel Universe, there's Kitty Pryde becoming a Spider-Man supporting character (which stems from Brian Michael Bendis' not-so-subtle obsession with her), getting most of her character development there as opposed to in the X-Men comics. In the main universe, the two barely know each other.
What other notable examples are there?