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 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.
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.
Well, it could be really wild. Maybe Steven could use his gem powers to try and stop Zim's crazy plans for world domination. Zim, on the other hand, would probably be super confused by Steven's powers and the whole concept of gems.
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.
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.
Definitely Dib and Dipper. They are both curious and into the strange and supernatural. Dib with his alien - hunting skills and Dipper with his knowledge of the Gravity Falls mysteries would make a great pair.
In such a crossover fanfiction, Gir could get into all sorts of trouble in Gravity Falls. He might mistake a Wendigo for a new type of snack. Meanwhile, Mabel could try to befriend Zim, thinking he's just a really odd - looking new friend. The story could involve a race against time as Zim's evil plans start to affect the delicate balance of Gravity Falls, and the local characters have to team up with Dib to stop him.