There might be a fanfiction where Gir crosses over into a different dimension within the Invader Zim universe. Maybe he meets alternate versions of Zim and the other characters. For example, he could find a version of Zim who has actually managed to conquer Earth, and Gir's actions in this strange new world could lead to all kinds of wacky adventures as he tries to find his way back to his own dimension.
One interesting Gir crossover Invader Zim fanfiction could be one where Gir teams up with Dib to stop some new alien threat that neither Zim nor the other regular villains can handle. In this story, Gir's unpredictable and silly nature might add a lot of comical elements to the usually tense relationship between Dib and Zim.
You can try websites like Archive of Our Own (AO3). It has a large collection of fanfictions, and you can easily search for Gir crossover Invader Zim stories. Just use the search bar and type in the relevant keywords.
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.
The clash of different aesthetics. Jhonen Vasquez has a very distinct style in both JTHM and Invader Zim. When you cross them over, you get a really unique visual and narrative blend.
One popular 'Invader Zim' female Irken fanfiction could be 'The Female Irken's Adventure'. It might explore the unique perspective of a female Irken in the context of the 'Invader Zim' universe. Maybe she has different goals or ways of approaching the mission compared to the male Irkens we usually see.
One interesting Invader Zim fanfic could be 'The Green Menace Returns'. It continues the story with Zim coming up with even more outlandish plans to take over Earth. Another good one is 'Dib's Discovery', which delves deeper into Dib's investigations of Zim's true identity. And 'Gir's Adventures' is also fun as it focuses on Gir's wacky antics around the city.
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 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.
Well, in 'Invader Zim' Gaz is a really cool and unique character. One interesting story could be about her constantly thwarting Zim's crazy plans in her own nonchalant way. For example, when Zim tries to take over the school, Gaz just goes about her day, but her mere presence sometimes messes up Zim's elaborate schemes because she's not easily intimidated or swayed by his antics.
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.
Another possibility is a fanfic set in an alternate universe where Gir and Zim are friends from the start. They go on adventures around the galaxy, with Gir's wacky antics often getting them into and out of trouble as they encounter different alien races and cultures.