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.
One idea could be to have Snake Eyes cross over into a modern urban setting where he has to use his ninja skills to fight a new kind of crime syndicate. Maybe he teams up with a local detective who is initially skeptical of his methods.
The key is to find common ground. Both have their own groups of heroes and villains. You can start by having a threat that affects both universes. For instance, a new evil force that wants to take over both Earth (where the Joes operate) and Cybertron (the Transformers' home). This forces the two groups to collaborate.
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.
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.
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.
One interesting gi joe fanfiction could be about a new recruit in the Gi Joe team facing unexpected challenges during a mission. For example, they might have to deal with a new type of enemy weapon that no one has seen before. Another could be a story that focuses on the relationships between the different characters outside of combat, like how they bond during their downtime at the base.
How about a crossover with Sonic the Hedgehog? Picture this: Sonic is running through a strange new zone and accidentally enters Joe's world. They join forces to stop Dr. Eggman who has somehow teamed up with one of Joe's enemies. Sonic's speed and Joe's Viewtiful powers would create a really fast - paced and exciting story.
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.