A story where the Powerpuff Girls' arch - enemy, Mojo Jojo, teams up with Zim. They both think they can use each other to achieve their own goals of world domination, but in the end, their different methods and egos cause their alliance to fall apart.
Maybe a fanfiction where Blossom, Bubbles, and Buttercup get accidentally teleported to Zim's planet. There they have to deal with the strange alien environment and Zim's nemesis, Dib, who sees them as potential allies in his fight against Zim.
One idea could be a crossover where Zim tries to take over Townsville and the Powerpuff Girls have to stop him. They initially underestimate Zim because of his small stature but soon realize his advanced alien technology is a real threat.
Here's a more detailed fanfiction idea. In a world where the Powerpuff Girls have been unable to stop a powerful new villain, they seek help from Dib, who then brings them to face Zim. Zim, thinking he can use this situation to his advantage, offers to help them. But in reality, he has his own plans to take over both Townsville and the Earth. As they work together, the Powerpuff Girls start to notice Zim's true intentions. Bubbles tries to befriend Gir to get more information, while Blossom tries to keep Zim in check. Buttercup is more skeptical and constantly on guard. Meanwhile, Dib is trying to document everything for his own records, hoping to finally prove Zim is an alien once and for all.
An interesting Invader Zim fanfiction could center around an alternate universe where Zim is actually a good guy sent to protect Earth from other threats. Or it could be about Zim and Dib getting stuck in a time loop and having to figure out how to break it. You might also consider a story that focuses on the strange and unique creatures in the Invader Zim universe and how they interact with Zim and Dib.
If Dib from Invader Zim were to meet the Powerpuff Girls, he'd be excited to find others who fight against the 'weird' and 'evil' forces. He'd probably try to convince them to help him expose Zim. The Powerpuff Girls might be hesitant at first, but if Dib shows them enough evidence of Zim's evil plans, they could team up.
One idea could be an OC who is a half - Irken, half - Earthling trying to find their place in both worlds. They could struggle with the Irken's evil plans while also trying to fit in on Earth. Another could be an OC from a different planet entirely who comes to Earth to stop Zim's chaos, believing it might endanger the whole galaxy. A third idea is an OC that is a mutated experiment on Earth that Zim mistakes for one of his own creations and tries to use in his schemes.
One idea could be that the Grimm from RWBY somehow end up in the Invader Zim universe. Zim, thinking they are some new form of alien threat, tries to study them while Dib sees them as a chance to prove himself by fighting them. Meanwhile, the RWBY characters are on a quest to get back to their own universe.
One idea could be a crossover with 'The Simpsons'. Zim could try to infiltrate Springfield, thinking it's a key Earth location for his invasion plans. Maybe he mistakes Homer for a powerful Earth leader and tries all sorts of wacky schemes to take over the town through him.
Gir could also be a great focus. His love for food and randomness could add a really fun element. Imagine Gir befriending the Powerpuff Girls and getting them into all sorts of silly adventures, while Zim is fuming in the background because Gir is not helping with his evil plans.
One plot idea could be that Zim discovers the power of the Sailor Scouts and tries to use it for his own evil plans. Maybe he thinks he can use their magic to finally take over Earth in a more powerful way.
One idea could be Zim getting sucked into a black hole and having to find his way out in a strange new dimension filled with bizarre creatures. He could use his usual over - confidence and wacky inventions to try and navigate this new world.
One interesting concept could be the exploration of Zim's character in a darker light. Maybe he's more menacing and his plans are even more devious. For example, his attempts to take over Earth could involve more cruel and calculated actions towards the humans.