Well, a cool concept for a Ben 10 Transformers crossover fanfiction could be that the Transformers' AllSpark has a strange effect on the Omnitrix. This makes Ben able to transform into new alien - Transformer hybrids. For instance, Four Arms could become a huge hulking form with robotic arms that can transform into different weapons. Also, the Decepticons might try to steal the Omnitrix to use its power to create more powerful soldiers. Meanwhile, the Autobots would team up with Ben to stop them. Maybe there could be a situation where Ben has to use his alien forms' unique abilities in combination with the Autobots' skills to defeat a powerful Decepticon threat.
An interesting plot for a Ben 10 Transformers crossover fanfiction might be that a group of Decepticons find a way to hack into the Omnitrix's technology. They start creating their own versions of Ben's aliens, but with a more evil and mechanical twist. Ben then has to work with the Autobots to not only stop the Decepticons from using these new creations for evil but also to find a way to fix the Omnitrix. Along the way, he might discover that there are some hidden functions in the Omnitrix that were put there by an ancient race that also had a connection to the Transformers' past. This could lead to Ben unlocking new and even more powerful alien forms that he can use to fight the Decepticons.
One idea could be that Lucifer, with his charm and manipulative skills, gets involved in the power struggles among the Autobots and Decepticons. He might try to use their war for his own amusement or to gain some sort of power. For example, he could offer a deal to Megatron to help him defeat the Autobots in exchange for something valuable in the underworld.
Maybe the Transformers end up in Remnant due to a space - time rift. The Grimm might be attracted to the Energon that the Transformers carry. The RWBY teams would have to team up with the Transformers to fight off both the Grimm and the Decepticons if they also show up. Yang could learn some combat moves from the Autobots, which would add a new dimension to her brawler style.
One idea could be that Sonic and his friends accidentally stumble into the world of the Transformers. Sonic's speed could be a valuable asset in a battle against the Decepticons. For example, he could quickly deliver important messages or parts between Autobots during a fight. Another idea is that the Chaos Emeralds somehow have an impact on the AllSpark, creating new powers or challenges for both the Sonic characters and the Transformers.
Another interesting concept is to have a character crossover. For example, Bumblebee from the Animated series could somehow end up in the world of the live - action Transformers movies. His small and agile form might be a big contrast to the more bulky Transformers in the movies. He could form an unexpected friendship with a human character, like Sam Witwicky, and together they could help the Autobots in the movieverse with a mission that requires his speed and intelligence.
Another idea is to have Ben 10 cross over with a futuristic sci - fi setting. His harem could be made up of female alien species he encounters. They could be intrigued by his ability to transform and his heroics, leading to a harem - like situation where they all vie for his attention.
Perhaps Ben teams up with the JLU to fight a common enemy that is immune to the Justice League's normal attacks but can be affected by some of Ben's alien forms. They could start off not trusting each other, but as the story progresses, they learn to work together and combine their strengths. Ben could use his alien forms like Four Arms to provide brute strength in battles, while the JLU members use their strategic skills.
One idea could be that Ben Tennyson's Omnitrix malfunctions while he's in the Avatar world. So, instead of transforming into his usual aliens, he starts taking on the forms of Avatar creatures. For example, he might turn into a flying bison or a waterbender - like spirit. This would create all kinds of chaos and new adventures as he tries to figure out how to control these new forms and also deal with the political and elemental challenges of the Avatar world.
One idea could be that Albedo creates a device that can swap powers with Ben, leading to a lot of chaos as they both struggle to adapt to the new powers. Another could be that Albedo and Ben are forced to team up against a new powerful enemy that threatens both their worlds. A third idea might be that Albedo gets stuck in Ben's body somehow and has to pretend to be Ben while dealing with his friends and family.
Another interesting concept is a character - based crossover. A Joe like Snake Eyes could form an unlikely bond with a Transformer like Bumblebee. Snake Eyes' ninja skills could be used in combination with Bumblebee's speed and agility in combat. They might start off not trusting each other but gradually learn to work together as they face various threats.
Perhaps there is a portal that connects Cybertron and Equestria. Some ponies accidentally end up on Cybertron and are shocked by the mechanical world. The Transformers have to help them get back home. Meanwhile, some Autobots are curious about the ponies' world and decide to accompany them. This leads to a cultural exchange of sorts between the two groups, with lots of misunderstandings at first but then growing respect and friendship.