One possible plot could be that Naruto and Ben Tennyson are somehow transported to each other's universes. Naruto, with his ninja skills, could be intrigued by Ben's alien forms and their powers. Ben, on the other hand, would be amazed by the chakra system in Naruto's world. They might team up to fight a common enemy that threatens both their worlds, like an alien force that has found a way to use chakra for evil purposes. This would lead to a lot of action -filled scenes where Naruto uses his jutsus and Ben transforms into different aliens to combat the threat.
One idea could be exploring the backstories of the aliens more in - depth. For example, how did Four Arms become so strong? Another could be a story where Ben has to team up with his enemies to fight a greater threat. Maybe Vilgax and Ben have to work together against an even more powerful alien race.
Well, aside from the main trio, characters like Vilgax are often included. He's a powerful villain and can create a lot of conflict in the story. Grandpa Max is also a good character to add. His wisdom and experience can guide Ben in difficult situations. And then there are the aliens themselves. For example, Rath is a very popular alien form of Ben, and fans like to write stories that focus on his wild and unpredictable nature.
Gara and Four Arms would have an interesting interaction. Gara, with his sand powers, is used to being in control and a bit of a loner. Four Arms is all about brute strength and action. When they meet, Four Arms might challenge Gara to a test of strength, which Gara would initially be hesitant about. But then they could find common ground in protecting their respective worlds. Four Arms could show Gara a more straightforward way of fighting, while Gara could teach Four Arms about using the environment to his advantage with his sand.
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.
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.
I'm not entirely sure as it could be a very specific or fan - made story. But generally in the Ben 10 Ultimate Alien series, Ben Tennyson has a new Omnitrix which gives him access to ultimate forms of his aliens. The big story might involve him facing new villains, protecting the Earth and the galaxy, and dealing with the consequences of using his powers.
Some of the alien characters themselves, like Heatblast or Four Arms, are also popular. Their unique physical characteristics and powers make them interesting to write about. For instance, Heatblast's ability to control fire can lead to all kinds of scenarios, from battling fire - based villains to using his heat to solve environmental problems in a story.
One common theme is friendship. The female alien often forms a strong bond with Ben, and they support each other through different situations. Another theme is power. The female alien usually has some unique power that plays an important role in the story.
I'm not entirely sure about the exact plot details of 'ben 10 ultimate alien the big story'. It might involve Ben Tennyson using his various alien forms to face some big threats and challenges, but that's just a general guess.