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.
The telepathic abilities in Young Justice, like Miss Martian's, could be very useful for the Totally Spies when it comes to getting information from enemies. In return, the spies' disguise and infiltration skills could help the Young Justice characters get into places they normally couldn't. They could work together in a mission where the spies pose as normal people and the Young Justice characters are there as backup, using their powers secretly when needed.
One idea could be that the Totally Spies are sent on a mission to investigate some strange paranormal activities. They then discover Danny Phantom's world and team up with him to stop an evil force that is using both magic and technology to take over the world. Danny could teach them about the Ghost Zone and they could share their spy skills with Danny.
In this fanfic, the setting could be a mysterious island. The Totally Spies are sent to investigate some strange energy readings. Meanwhile, Martin Mystery and his team are there looking into rumors of a cursed treasure. When they meet, there's some initial tension. However, as they start exploring the island further, they find that the strange energy is related to the cursed treasure. The Totally Spies' ability to hack into security systems and their combat training comes in handy, while Martin Mystery's ability to decipher ancient inscriptions and deal with the supernatural helps them understand the true nature of the curse. Together, they face off against a group of pirates who also want the treasure and the evil spirit that guards it.
One possible plot is that a mad scientist creates a device that can manipulate time. This device falls into the wrong hands and starts causing chaos in both Gotham and the places where the Totally Spies operate. Batman and the spies get caught in the time - warped events. They have to figure out how to reverse the effects while also dealing with the changes in the timeline that are making their enemies stronger. The fanfic could explore how they adapt to these changes and use their unique abilities to save the day.
One idea for a Spiderman and Totally Spies crossover fanfic could be that the Totally Spies are on a mission in New York. They accidentally stumble upon one of Spiderman's battles with a villain. Spiderman is initially suspicious of them but soon realizes they have the same goal of protecting the city. They then team up, with the Totally Spies using their high - tech gadgets and Spiderman his superpowers to take down the bad guys.
A possible plot could be that there is a threat to the world that involves both superpowers and high - tech espionage. The Totally Spies are called in to investigate the technological aspect, while Spiderman is already on the scene dealing with the super - powered part of the threat. They cross paths and at first are not sure if they can trust each other. But as the situation worsens, they realize they need to work together. They combine their unique skills, like Spiderman's web - slinging and the Totally Spies' ability to blend in and use advanced gadgets. In the end, they defeat the common enemy.
It could be the Joker. Imagine the Joker getting involved in a global scheme and teaming up with some international bad guys. His chaotic nature would be a huge challenge for both Batman and the Totally Spies. He would use his usual tricks like laughing gas and mind games, but on a much larger scale.
A crossover with Sailor Moon might also be cool. The Totally Spies could find themselves in the Sailor Moon universe and have to help the Sailor Scouts fight against a new enemy that is after some powerful artifact in both their worlds. The spies' espionage skills would be useful in gathering information about the enemy's plans, while the Sailor Scouts' magical powers could be used in direct combat.
In a crossover with Danny Phantom, Danny would be a key character. His ghost - fighting abilities and the Totally Spies' skills in dealing with more earthly threats could combine in an interesting way. For example, they could face a villain who has the ability to phase between the ghost zone and the human world, and Danny and the spies would have to use their respective skills to stop him.