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.
Another fanfic concept could be that the Young Justice team discovers the existence of the Omnitrix and tries to study it. Ben, being protective of his device, initially clashes with them but then realizes they have similar goals of protecting their respective worlds. So, they join forces. Ben could even show them some of his alien forms and how to use their powers more effectively in combat.
One idea could be a crossover where Ben Tennyson ends up in the Young Justice universe. He could team up with the young heroes to fight a common enemy. Maybe a group of villains from both universes join forces, and Ben uses his Omnitrix to transform into different aliens to help in the battles.
One idea could be that the Young Justice team has to prove themselves to the Justice League during a major alien invasion. The League initially doubts their abilities but as the battle progresses, they see the unique skills and teamwork of the young heroes.
Well, the plot could involve Ben Tennyson and the Young Justice team teaming up to fight a common enemy like Darkseid. Ben could use his various alien forms to complement the powers of the Young Justice members. For example, Heatblast could help Superboy when dealing with cold - based threats.
First, you need to do your research on both universes. Know the characters, their powers, and the overall storylines. For example, in Ben 10, Ben has a wide range of alien forms with different abilities, and in Young Justice, each hero has their own unique set of skills and backstories. Make sure you're true to these elements when writing.
One great plot idea could be that the characters from Young Justice and Totally Spies are brought together by a common enemy. Maybe a powerful villain has a plan that affects both their worlds, like a device that can mind - control both superheroes and spies. So they have to combine their skills to stop it.
A cool plot could be that a dimensional rift opens up and pulls some of the Young Justice characters into the Ben 10 universe. They then have to find a way to get back home. Along the way, they meet Ben and join forces with him. Ben shows them around his world, and they help him deal with some local threats. For instance, Aqualad could use his water - based powers in conjunction with Ben's Four Arms in a fight against a group of robotic enemies that are causing chaos in a city near Bellwood.
Maybe there's a multiverse - related plot where the universes of Spider - Man and Young Justice start to merge. This causes chaos as characters from both worlds start to appear in the wrong places. The Young Justice team and Spider - Man have to work together to find a way to separate the universes again. They'd face challenges like dealing with different power levels and the cultural differences between their two groups.
In 'Ben Ten and Young Justice fanfic', a major theme is adaptation. Ben has to adapt to the rules and ways of the Young Justice world, and the Young Justice characters have to adapt to having Ben around with his strange powers. Another common theme is the discovery of new abilities. Ben may discover new uses for his Omnitrix in the context of the Young Justice universe, and the team may discover new aspects of their own powers through interacting with Ben.
One idea could be that Ben Tennyson, while on a mission in another dimension, accidentally lands in the X - Men's universe. His Omnitrix starts acting strangely due to the different energy signatures there. The X - Men initially see him as a threat but then realize he could be an ally in fighting a common enemy, like a powerful mutant - alien hybrid that is causing chaos. Ben could transform into some of his alien forms to help the X - Men in battles, and there could be interesting interactions between the different characters, like Gwen and Rogue comparing their powers.
One popular idea is crossovers with other popular animated shows like Teen Titans. It's exciting to see the Ben 10 characters interact with the Titans. Another idea could be a crossover with the Powerpuff Girls, imagining how the different powers and personalities would clash or cooperate. Also, crossovers with the Justice League can be really cool, as Ben 10's alien forms could bring a new dynamic to the superhero team.