In a crossover with 'Powerpuff Girls', the Professor could be an interesting character to include. He might study Zim's technology and try to find a way to use it for good, while the Powerpuff Girls are busy fighting Zim's minions. And Zim would be both fascinated and enraged by the super - powered girls.
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.
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.
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.
You could include characters from other crime - themed games like Tommy Vercetti from Grand Theft Auto: Vice City. His experience in the crime world could create some interesting dynamics with the Mafia 3 characters. They could either be rivals or form an uneasy alliance.
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.
There's also 'Ayam's Encounter with Invader Zim'. In this fanfic, Ayam is a new character who stumbles upon Zim's base and what follows is a series of hilarious and action - packed events as they interact.
In the 'Invader Zim Graphic Novel', the central characters are Invader Zim and Dib. Invader Zim is a short - tempered and somewhat clueless alien who believes he can easily take over Earth. Dib, on the other hand, is a bit of an outcast among humans but is highly intelligent and obsessed with uncovering Zim's alien identity. Additionally, there are supporting characters like Gir, Zim's malfunctioning robot sidekick, who adds a lot of comical elements to the story.
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.
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.
Definitely Deadpool. His chaotic and fourth - wall - breaking nature would add a whole new level of craziness to the Suicide Squad. He could have some hilarious interactions with Harley Quinn, like competing for the 'craziest character' title. And his regenerative abilities would make him a valuable asset on dangerous missions.