Since Marvel's inception in the 1960s, numerous of its characters have appeared alongside "The Worlds Greatest Superheroes" of DC Comics. These titles are always one-shots or a mini-series telling stories outside of both companies' regular continuity.
Even with the constant threat of Marvel leveraging the power of Disney to purchase its distinguished competitor, the idea of heroes from both publishers appearing within the pages of a regular monthly title seems almost impossible. The JLA and Avengers could only appear in a series of "special issues" both companies would publish. The idea that Superman could "turn up" in Uncanny X-Men #444 is beyond inconceivable to most fans.
Starting in the 1990s, a bizarre set of circumstances, including desperate publishers, superstar artists, corporate acquisitions and the "death" of some of Marvel's biggest franchises, would ultimately result in a whole "universe" worth of characters having existed in both Marvel and DC continuity.
This story is not an "Else-worlds", a "What if....?" or a "Squadron Supreme" type of pastiche. It's just a strange four-issue Marvel story arc that both companies have chosen to forget.
8. The Image Exodus
In one of the most famous events in the medium's history, seven of Marvel's top creators walked out to start their own comic book company. Fed up with seeing Marvel profiting off characters they had created, Jim Lee, Rob Liefeld, Todd McFarlane, Erik Larsen, Marc Silvestri, Jim Valentino and Whilce Portacio struck out on their own and formed Image Comics.
The company's goal was to champion the creators' rights to such an extent that the only thing that Image Comics would own was the company logo.
Although initially the characters would exist in a shared universe, internal squabbling between the partners made this particular aspect of the new company fall by the wayside; leaving each creator in sole control of their studio and free to do whatever they wished with their intellectual properties - be it start a toy company, create an entire universe of titles or take their ball and go home.
Despite their best intentions, it didn't take long for the cracks in the partnership to start to show.