In the "Doomsday Clock" event from DC Comics, the DC Universe and the world of "Watchmen" (by Alan Moore, Dave Gibbons, and John Higgins) collide. The comic sees the all-powerful Doctor Manhattan rewrite the DC Universe as it stands on the brink of collapse due to the rise of metahumans.
In "Doomsday Clock" #12, after initially looking into the past, Doctor Manhattan looks into the future and sees many different occurrences: In 2020, Superman's timeline is bombarded by the Old Gods, potentially referring to Superman's encounter with the Old God Olgrun in "Action Comics" (by Phillip Kennedy Johnson, Riccardo Federici, Will Conrad, Lee Loughridge, and Dave Sharpe). In 2025, a crisis unlike any other erupts across the Metaverse, with Superman coming out of it revitalized. In 2026, the timeline gets restored and Earth-5G is born, while Superman searches for Bruce Wayne's lost daughter to save his son. Most shockingly, in 2030, a "Secret Crisis" begins with a universe sounding a lot like Marvel's fighting against and alongside DC's.
"On July 10, 2030, the 'Secret Crisis' begins, throwing Superman into a brawl across the universe with Thor himself ... and a Green Behemoth stronger than even Doomsday, who dies protecting Superman from these invaders," the synopsis reads.
While the Marvel Universe isn't named, the comic directly references Thor and hints at the Hulk, with the latter referred to as a "Green Behemoth stronger than ever Doomsday." In the future that Doctor Manhattan sees, the Hulk dies, protecting Superman from the attackers. The namedrop of Thor and the Hulk teases are the only mentions of Marvel heroes in "Doomsday Clock," but they still can set up a potential future crossover. However, there are some obstacles to making it happen.
