Maybe there's a device in Danny's world that has been stolen and is being used in the Totally Spies' world to cause chaos. The spies and Danny have to work together to track it down. The spies would be great at the detective work on their end, while Danny can use his ghost powers to sense the device's otherworldly energy.
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 crossover, the villains from both shows could team up. The evil ghosts from Danny Phantom and the bad guys from Totally Spies might join forces. So, Danny and the Totally Spies need to put aside their differences in their fighting styles and worlds to defeat this new, combined threat. Danny can fly around and be invisible to gather intel, while the spies use their gadgets and martial arts skills in the fight.
Danny would probably be in awe of the Totally Spies' high - tech equipment at first. He might try to figure out if he could use some of their tech to enhance his ghost - fighting abilities. Meanwhile, the spies would be intrigued by Danny's ghost powers. They could ask him to teach them how to turn invisible like he does. And Danny could learn some hand - to - hand combat skills from the spies.
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.
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.
For the crossover, a good plot could be that a new type of ghost appears that is immune to Danny's ghostly powers. The Ghost Hunt team, which has encountered a similar situation before, offers their expertise. They introduce Danny to a special type of ectoplasm - based weapon that can harm this new ghost. Danny then teaches them some of his combat moves as a half - ghost to better fight the ghost.
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.
Another idea is a time - travel lemon fanfic. The spies travel back in time to a different era, like the 1960s. There, they face new challenges and also develop different relationships. Maybe one of the spies falls in love with a character from that time period and there are some steamy moments as they navigate the differences in culture and technology.
I like the idea of a Danny Phantom and Ben 10 crossover. Ben has the Omnitrix which gives him access to different alien forms, and Danny has his ghost powers. They could either be rivals at first, each thinking their powers are better, or they could quickly team up to face a threat that requires both their unique abilities. It would be a fun clash of two very different power - based universes.