Another possibility is that all four of them - Dib, Dipper, Zim, and Mabel - could be main characters. They would each bring their own unique personalities and goals. Dib wants to stop Zim, Dipper wants to protect Gravity Falls, Zim wants to conquer, and Mabel just wants to have fun and make friends. Their interactions would drive the story forward in all sorts of unexpected ways.
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.
Zim and Mabel could also be main characters. Zim with his over - the - top plans to take over (or in this case, take over Gravity Falls) and Mabel with her positive and somewhat naive attitude. She might try to show Zim the good in Gravity Falls while Zim is just thinking about how to rule it. And of course, Gir would be around causing chaos as usual.
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.
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.
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.
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.
There could also be other characters from the original Invader Zim series popping up in the fanfiction. For instance, Gir, Zim's robot sidekick. Gir's antics can often provide comic relief and also play a role in the relationship between Zim and Gaz. If Gir likes Gaz or is scared of her, it can create interesting situations that drive the plot forward.
Mabel and Hilda would be fast friends. They both have a love for the strange and wonderful. Mabel would be excited to show Hilda her sweaters and all the cool stuff in the Mystery Shack. Hilda would share her adventures in Trollberg with Mabel. They might even come up with some new and crazy ideas for adventures together, like having a slumber party in a haunted forest. Wendy would probably be a bit more aloof at first, but as she gets to know Hilda, she'd start to warm up and they could go on some more adventurous outings, like climbing a really tall mountain in Hilda's world.
Well, I think Lance would be really intrigued by Mabel. He'd probably be impressed by her colorful sweaters and her positive attitude. Mabel, on the other hand, would be excited to meet a real - life space hero like Lance. She might even try to set him up with one of her friends in Gravity Falls.
Bill Cipher would be a major wildcard in their interactions. If he were to cross paths with the Winchesters, he would likely try to manipulate them for his own amusement. He might offer them false information or try to pit them against the other characters in Gravity Falls. The Winchesters, being used to dealing with devious creatures, would have to be on their guard and figure out a way to outwit Bill while also trying to solve whatever mystery has brought them to Gravity Falls in the first place.
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.
The main characters would likely be Gaz and Zim. Gaz, as the one with the time machine, would be central to the story. Zim would also be important as he'd probably be trying to either get the time machine from her or interfere with her time - traveling adventures.