Batman could be another interesting crossover. Batman's dark and brooding nature would contrast with Cage's more flamboyant style. They could be forced to team up to solve a mystery that involves some sort of magic or otherworldly elements, which would be new territory for Batman and something Cage is more used to in the Mortal Kombat realm.
I think Deadpool would be a great crossover character with Johnny Gat. They both have a crazy sense of humor and are not afraid to get into some wild and violent situations. Their banter would be hilarious as they fight side by side against bad guys.
His character could also be developed through learning new fighting styles. In the crossover, he might encounter a Mortal Kombat master who teaches him a more disciplined and deadly form of combat. This not only makes him a more formidable fighter but also changes his attitude towards fighting. He becomes more respectful of the Mortal Kombat traditions and incorporates them into his own flamboyant style.
How about a crossover with the 'Teenage Mutant Ninja Turtles'? Johnny Cage could end up in New York City's sewers and meet the turtles. He'd be initially shocked but then impressed by their ninja skills. They could all work together to stop Shredder from getting his hands on some powerful ancient artifact that Cage was also after.
The sorceress Sindel could also be a main antagonist. She may believe that the relationship between Johnny and Mileena goes against the natural order of things. She could use her powers to create illusions and misunderstandings between them. Also, she might be trying to protect some ancient Outworld secret that she thinks Johnny Cage is getting too close to through his relationship with Mileena.
Maybe Johnny Cage is suddenly part of a Mortal Kombat tournament in a crossover. He uses his acting skills not just for show but as a strategic advantage. For example, he could pretend to be injured to lure opponents into a false sense of security and then strike back with a powerful combo. His showbiz connections could also play a role, like getting access to some special weapons or training techniques that no one else in the Mortal Kombat world has thought of.
Superman from the DC universe. Superman stands for truth, justice, and the American way, but his methods are often more lenient compared to Akainu. The two could have a philosophical debate about justice while also having to fight a common enemy.
His best friend. This person might be by his side throughout his illness, bringing him comfort, and trying to keep his spirits up. They could also be the one who helps Johnny find the best doctors or alternative treatments.
Luffy from 'One Piece' would also be an interesting crossover character. Imagine Ichigo being abandoned on an island in the 'One Piece' world. Luffy's carefree yet determined nature could contrast with Ichigo's more brooding personality. They could embark on a journey together, with Ichigo learning about the concept of the 'One Piece' and using his powers in unexpected ways within this new world.
Kakashi from Naruto himself could be part of a 'grey' themed crossover within the Naruto universe. Maybe a situation where Kakashi's past and his more mysterious side are explored further, and Naruto has to work with him in a darker, more complex mission that delves into the grey areas of the ninja world.
Alice from Alice in Wonderland could also be an interesting choice. Her curious nature would lead her to explore the world of Young Dracula. And Young Dracula might be both amused and a bit annoyed by her constant questions and her fearless attitude in the face of his vampiric self.