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.
Blossom would be an interesting character to focus on. She's the leader of the Powerpuff Girls and very intelligent. In a fanfiction, she could be the one who tries to outwit Zim. She might analyze his every move and come up with plans to counter his schemes. At the same time, she could also learn from Zim's advanced technology and use it to enhance the Powerpuff Girls' powers in some way.
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.
Ruby and Zim could be an interesting pairing. Ruby is young and full of energy and optimism, while Zim is completely delusional and self - centered. Their interactions would be full of misunderstandings and comical moments as Ruby tries to understand Zim's alien ways and Zim just sees Ruby as another obstacle in his plan to conquer Earth.
Zim and Bender would be a really interesting interaction. Zim is this over - confident alien bent on world domination, while Bender is a self - centered, kleptomaniac robot. They could start off by trying to out - do each other in terms of causing chaos or stealing things. GIR and Fry could also be a fun pair. Fry's simple - mindedness would match well with GIR's random and silly behavior. They could get into all sorts of comical situations together, like accidentally starting a food fight in the Planet Express cafeteria.
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.
I think Steven and Zim would be the most interesting pair. Steven's kind - hearted and positive nature contrasted with Zim's evil and self - centered personality would create a lot of comical and tense moments.
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.
Sonic would be really interesting. His super - speed and adventurous nature would contrast well with the Powerpuff Girls' more home - based superheroics in Townsville. His encounters with the villains in Townsville would be full of action as he zooms around trying to stop their evil plans, all while trying to figure out how to get back to his own world.
Dib is also very popular. His obsession with proving Zim is an alien and his determination, despite being constantly ridiculed, give him a lot of depth. Fanfiction often explores his relationship with his sister and his father in more detail, adding to his character.
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.
One popular theme is the relationship between Zim and Dib. Writers often explore their rivalry in different ways, like what would happen if they were forced to work together. Another common theme is the exploration of Zim's alien culture and how it clashes with Earth's. It's interesting to see how different authors handle these cultural differences.