WARNING: The following contains major spoilers for The Rise of Ultraman #2 by Kyle Higgins, Mat Groom, Francesco Manna, Espen Grundetjern, and VC's Ariana Maher, on sale now.
The latest entry into the Ultraman franchise, The Rise of Ultraman, follows a very similar formula that any fans will easily recognize. Ultraman bonds to a human host and joins the U.S.P. in their fight against interdimensional Kaiju and extraterrestrial attacks. This time around might not see the threat contained to just the Ultraman universe, but quite possibly the primary Marvel 616 universe. It might sound like speculation based on a subtle nod in The Rise of Ultraman #2, but the precedent is already being set elsewhere.
In The Rise of Ultraman #1, Cadet Kiki Fuji shows up to work at the United Science Patrol, a secret international organization dedicated to protecting humanity from the kaiju threat. When on a field mission with Captain Muramatsu, the two are caught off guard and underprepared while facing off with a kaiju but are saved by Kiki's friend Shin Hayata. Shin is desperate to join in the fight, and has been building his own tech to help locate and fight against kaiju, but didn't make the cut when he attempted to join the U.S.P. alongside Kiki. That evening while the two old friends discuss this over dinner, Kiki receives an emergency alert from Captain Muramatsu, and Shin drives her to the sight. It's in the middle of a forest clearing that they encounter a glowing red sphere embedded in the middle of a crater.
Captain Muramatsu tells Kiki and Shin that this is the second recorded encounter with an extraterrestrial, as the kaiju hail from another dimension. Another UFO encounter supposedly killed one of the U.S.P.'s best agents back in 1966, and the robot that emerges from the sphere sets Muramatsu immediately on the offensive, giving Shin the order to shoot. When Shin finds empathy for the Ultra, the two touch hands, and both are engulfed in a blinding light.
It's during Rise of Ultraman #2 that it's learned that Shin and the Ultra are themselves becoming intertwined on a metaphysical level. As they experience one another's memories, the Ultra explains to Shin that their merging process has been compromised, and if they are to be successful in the endeavor then Shin needs to give up control of his body. This is, understandably, a problem for Shin, as he doesn't understand why he should be the one to concede control. Shin understands that the kaiju are a threat, but it's what the Ultra tells him about them that not only truly shocks him, but also sets up the Ultraman universe for a potential crossover with the greater Marvel universe.
The Ultra explains that the kaiju are empowered and introduced to the universe by a mysterious energy "from beyond the known." This energy is drawn to negative emotions such as greed, jealousy, and hate, specifically mentioned by the Ultra as "darkness of the soul." His people became referred to as "Ultra" by ascending beyond the reach of the kaiju, and spent the rest of their time trying to show others how to do the same as well. This is key to the Ultra, because as Shin is told, "they are not unique to your world. They exist across the multiverse..."
This particular phrase is key to any possible crossover event. The main Marvel universe is designated as 616, but that is just part of the Marvel Multiverse, across which an almost infinite number of realities exist, including the Marvel Zombies universe (2149), and the Ultimate Marvel universe (1610). If The Rise of Ultraman now fits under that umbrella, it wouldn't be surprising to see this and the 616 universes collide, especially since Deadpool is already experiencing a monster crisis of its own.
While the U.S.P. fights kaiju in the Ultraman universe, Deadpool is currently the King of Monsters, and Manhattan, in the 616 universe. It might not be a long-lived reign, but Deadpool's subjects are specifically monsters, not mutants, not aliens. Ultraman is already more or less a sci-fi version of Monster Hunter, and Deadpool is dating one of those right now. Plus, if there is any Marvel property that could get away with interdimensional giant robots and monsters, it's probably Deadpool.