Well, it could be about a space girl who encounters Invader Zim. Maybe she tries to stop his evil plans on Earth. She might use her special powers from outer space to outwit Zim and his various gadgets.
Obviously, the space girl and Invader Zim would be the main characters. There could also be Gir, Zim's clueless but lovable robot sidekick.
It could be about Zim finally succeeding in his plan to take over Earth. Maybe he discovers a new weakness in the humans that he exploits.
One possible plot could be that Zim and Dib from 'Invader Zim' somehow end up in the 'Futurama' universe. Zim might see the advanced technology in Futurama as a new opportunity to take over a planet. He could mistake Planet Express for a major power center and try to infiltrate it. Dib, on the other hand, would be excited to explore this new world and might team up with Fry, Leela, and Bender to stop Zim's new evil plan.
One popular Invader Zim girl fanfic might be those that focus on a female character's adventures in the strange world of Invader Zim. For example, a story where a girl discovers Zim's true identity and has to decide whether to expose him or help him in his bumbling attempts at world domination.
Often, there's a theme of self - discovery. As Female Zim is a different take on the character, she might be on a journey to figure out who she is in this new form. Also, the theme of adaptation is common. Just like in the original where Zim has to adapt to Earth, Female Zim may have her own unique ways of adapting, whether it's to Earth's environment or to the reactions of the other characters around her. And there's also the theme of power. How does being female change Zim's perception and use of power? This is something that many fanfictions like to play with.
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.
No, they aren't. The Invader Zim comics have completed their run for now.
There could be a new Irken who is sent to Earth to spy on Zim and make sure he is doing his job properly. This new character might have a different appearance and personality compared to Zim, perhaps being more calculating and less impulsive.
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.