A fanfiction could involve Zim getting mistaken for an alien criminal by the Men in Black. Ms. Bitters, being her usual creepy self, somehow gets involved in the chaos. She might try to use the situation to her advantage, perhaps by making a deal with the Men in Black to get rid of Zim once and for all. Meanwhile, Zim is trying to prove that he's on Earth for his own 'evil' purposes and not the ones the Men in Black suspect.
Well, Zim could be seen as a potential threat by the Men in Black. They might approach him thinking he's just another alien causing trouble on Earth. Ms. Bitters, on the other hand, could be like a wild card. She might try to protect Zim because she has her own plans for him, or she could sell him out to the Men in Black. It really depends on her motives in the fanfiction.
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 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 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.
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.
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.
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.
A fanfiction could revolve around a spell gone wrong in the Harry Potter world that accidentally transports Ms Marvel there. She then has to navigate the wizarding world while also trying to find a way back home. Along the way, she teams up with Hermione to study the magic that brought her there and in the process, they learn from each other's worlds.
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.