I'm interested in situations where fictional universes have merged together or split apart.
The DC Universe has many examples of this.
Fawcett Comics licensed Captain Marvel and his associated characters to DC in 1972 and Captain Marvel was fully integrated into the DC Universe in 1985.
Charlton Comics characters including Blue Beetle, Peacemaker, The Question and Peter Cannon were also integrated into the DC Universe in 1985. Peter Cannon, however, stopped appearing in DC Comics after the early 90s and is now being published by Dynamite Entertainment.
Milestone characters were always published by DC Comics, but existed in a separate universe. In 2009 a Justice League of America story arc placed them in the DC Universe as did the Young Justice animated series. They are back to being their own separate universe after the 2020 Milestone relaunch.
Wildstorm characters were originally part of the Image Universe when their series began in 1992. During this time, crossovers with other Image properties like Cyberforce and Savage Dragon occurred frequently. But after the Shattered Image crossover in 1996, Wildstorm became its own separate universe that had occasional crossovers with the DC Universe despite the fact that DC wasn't supposed to have a multiverse at the time. In 2011, it became fully integrated with the DC Universe.
While Spawn is technically still part of the Image Universe, crossovers became infrequent after the 90s. Spawn supporting character Angela was bought by Marvel Comics and integrated into the Marvel Universe, which retconned her to be the daughter of Odin.
A version of Conan the Barbarian, Godzilla, Fu Manchu, Rom and the Micronauts have been part of the Marvel Universe while Marvel held the license, but now Marvel is no longer able to refer to them or even reprint their appearances.
Lady Death was one of the main characters of the Chaos! Universe in the 90s, but was later split into her own universe in the 2000s, first by Avatar Press, later by Coffin Comics. When the Chaos! Universe was rebooted by Dynamite Entertainment, Lady Death was replaced by Lady Hel to plug the holes in continuity that would've been caused by Lady Death's absence. In the 2020s, the Chaos! characters were fully integrated into the Vampirella Universe.
In 2016, Transformers, G.I. Joe, Rom, Micronauts and M.A.S.K. became united as the Hasbro Universe, despite the fact that the earlier Infestation miniseries presented Transformers and G.I. Joe as existing in separate universes. When IDW lost the license to these characters, the Hasbro Universe was rebooted by Image Comics, this time only including Transformers, G.I. Joe and a new property called Void Rivals.
Formerly creator-owned superheroes Wildstar and Ant were eventually integrated into the Savage Dragon Universe after Erik Larsen bought them. Public domain superheroes such as the Bart Hill version of Daredevil also became part of the Savage Dragon Universe. However, these heroes sometimes make appearances in other universes due to being public domain.
Speaking of public domain, Dracula, Frankenstein's Monster and the Greek gods are integral parts of the Marvel universe where events very similar to the Dracula novel, the Frankenstein novel and the Iliad have taken place. The Greek gods and Frankenstein are also part of the DC Universe.
What do you guys think about the idea of universes incorporating characters from other universes or splitting apart? Do you like it or does it make continuity hard to follow? Are there any examples that I missed? What universes do you think will merge or split in the future?