In a fanfiction crossover, they could end up in each other's universes by accident. Ben finds himself in a world full of EVOs and has to rely on Rex to get back home. Meanwhile, Rex is intrigued by the Omnitrix and tries to figure out how it works. They face many challenges together, like dealing with Rex's old enemies who see Ben as a new target.
There could be a plotline where an ancient artifact is discovered that has the potential to either destroy or save both the world of Ben 10 and Generator Rex. Ben and Rex are drawn to it for different reasons. Ben thinks it could be a powerful weapon against his enemies, while Rex believes it could help him understand more about the origin of nanites. However, they soon find out that there are many forces trying to get their hands on this artifact too. They must protect it from falling into the wrong hands and also decide what to do with it in the end. This would involve a lot of action, adventure, and moral dilemmas for both characters.
Heatblast is really cool. He can generate and control intense heat, which makes him great for combat and for melting things. He has a flaming body which gives him a very intimidating look.
Fourarms is really cool. He's big and strong with four arms, which gives him great power in combat. Heatblast is also amazing as he can generate and control fire.
At first, they could be rivals. Ben might think Rex's nanites are too unpredictable, and Rex could see Ben as a bit of a show - off with his Omnitrix. But as they face common enemies, they start to respect each other. For example, when they're both captured by a powerful villain, they see how the other fights and starts to understand their strengths.
One interesting crossover could be their powers combining in a unique way. For example, Ben could use one of his alien forms' abilities in tandem with Rex's nanites. Maybe Heatblast's heat powers could supercharge Rex's nanite - based weapons. Another aspect could be their personalities clashing and then learning from each other. Ben, being more of a carefree hero at times, could teach Rex to loosen up a bit, while Rex's more serious and scientific approach could help Ben be more strategic in his battles.
Gwen would be a great character to pair with Alien X. Her magical abilities could complement Alien X's powers in interesting ways. For instance, when facing a threat that requires both brute omnipotent force and the finesse of magic. Another good pairing could be Kevin. His ability to absorb and use different forms of energy could interact with Alien X's power source in unexpected ways, like when they need to combine their powers to break through a powerful energy barrier.
Another plot could be that an interdimensional rift opens up, and it's causing chaos in both Ben's and Rex's worlds. They discover that the source of the rift is related to the technology of both the Omnitrix and the Nanites. Together, they journey through the rift to find the cause and close it. Along the way, they meet alternate versions of themselves and their friends, which adds an interesting twist to the story.
It could be Ben when he transforms into Alien X. Alien X has near - omnipotent powers. He can manipulate reality on a cosmic scale. In a crossover, he could potentially use those powers to control the Nanites or even restructure Rex's powers in some way. However, he's also very difficult to control, so it's a double - edged sword.
I think an evil scientist who has found a way to combine the power of the Omnitrix and the nanites in a wrong way could be really interesting. He could be creating hybrid monsters that are very difficult for Ben and Rex to defeat. He would be constantly one step ahead because he understands both technologies so well.