One interesting concept could be the combination of the unique alien forms in Ben 10 with the dark and complex lore of Warhammer. For example, how would an Omnitrix - transformed alien interact with the various factions in Warhammer, like the Space Marines or the Orks? Another concept might be the power dynamics. Ben's powers are usually used for good, but in the Warhammer universe, the lines between good and evil are often blurred. So, it would be fascinating to see how he navigates this new moral landscape.
Well, Ben is the central figure. But don't forget about Kevin Levin. His ability to absorb and use different forms of energy could be very relevant in the Warhammer universe filled with all kinds of strange energies. In Warhammer, a Chaos Sorcerer could be a main character. The way his dark magic would interact with Ben's powers and the overall plot would be full of potential. Also, Rook Blonko from Ben 10 could be important. His skills and loyalty could add a different dynamic to the story as they explore the Warhammer universe together with Ben.
One interesting concept could be a 'Warhammer' crossover with 'Lord of the Rings'. Imagine the orcs from 'Warhammer' interacting with the orcs from Middle - earth. Another could be a crossover with 'Star Wars', like the Space Marines teaming up with the Jedi or Sith in some strange alternate universe scenario. And a 'Warhammer' and 'Harry Potter' crossover where the magic of the wizarding world meets the dark and gritty world of 'Warhammer' would also be really cool.
First, choose the two universes you want to cross over. For example, if it's Warhammer 40k and Star Trek. Then, research both universes thoroughly. Know the lore, the characters, the technology, etc. After that, come up with a basic plot. Maybe the Starfleet encounters the Imperium of Man and there's a misunderstanding that leads to conflict at first.
One idea could be to have Ben Tennyson end up in the Young Justice universe by accident. Maybe his Omnitrix malfunctions during a fight and transports him there. He could then team up with the Young Justice heroes to fight a common enemy like the Light or some new villain that's a threat to both universes.
In a Ben 10 and Naruto fanfic, their powers would be a great combination. Ben has the ability to transform into various alien forms, each with unique powers. Naruto has his chakra and his powerful jutsu. They could learn from each other. For example, Ben could teach Naruto about some of the alien technologies, while Naruto could teach Ben about chakra control and the ninja way.
You can start by looking on fanfiction.net. It has a large collection of fan - made stories, and there are likely to be some great 'warhammer crossover fanfic' there. Another good place is Archive of Our Own (AO3). It has a well - organized tagging system, so you can easily search for 'warhammer' crossovers with other fandoms.
One idea could be a crossover with Star Wars. Imagine the Imperium of Man encountering the Galactic Empire. The two totalitarian regimes could have some intense ideological battles and epic space battles as well.
How about a crossover with 'Final Fantasy'? The different job classes in Final Fantasy could be compared to the various character types in Warhammer Fantasy. For instance, the Black Mages in Final Fantasy could team up with the Warhammer Fantasy mages to cast powerful spells against a great evil. And the idea of having Warhammer Fantasy's Chaos forces corrupt some of the beautiful and idyllic worlds in Final Fantasy could create a really dark and engaging story.
Well, one big challenge is getting the lore right. Warhammer has a ton of detailed lore, and Young Justice also has its own established universe. Combining them without messing up the facts is tough.