Definitely include Bill Cipher from Gravity Falls as he is the main villain in Weirdmageddon. Dipper and Mabel Pines are also essential as they are the main protagonists in the Gravity Falls story. If it's a crossover with the Over the Garden Wall universe, Wirt and Greg would be interesting additions. Wirt's cautious and somewhat gloomy nature could contrast with the more adventurous Gravity Falls characters, while Greg's innocence could add a different dynamic.
If you're doing a crossover with the Regular Show, Mordecai and Rigby would be great. Their comical and often irresponsible behavior could lead to some funny situations in the middle of Weirdmageddon. They could learn from the more serious Dipper and Mabel. Also, Benson could be there to try and keep some order among the chaos. Another good character to include could be Eda from The Owl House. Her powerful magic could be a real game - changer in the fight against Bill Cipher.
In a crossover with the Amphibia universe, Anne, Sprig, and Polly would be good choices. Anne's martial arts skills and her growth throughout her own story could be valuable in Weirdmageddon. Sprig's naivete and courage, and Polly's sassiness could add some fun elements. They could all interact with the Gravity Falls characters in unique ways, like Anne teaching Dipper some self - defense moves, and Sprig being amazed by the strange creatures in Gravity Falls.
One idea could be a crossover with the Gravity Falls and Rick and Morty universes. In this fanfiction, Rick could end up in Weirdmageddon and try to outwit Bill Cipher using his advanced technology and cynical attitude. Morty might befriend some of the Gravity Falls characters like Dipper and Mabel, and together they face the chaos of Weirdmageddon.
If it's a crossover with the Legend of Korra, Korra herself would be an excellent addition. She is a powerful bender and has a lot of experience in fighting evil. She and the Trollhunter could learn from each other's fighting styles. Asami could also be included, providing her engineering skills to help the Trollhunter with any technological needs in their battles against the common foes.
For a crossover with a post - apocalyptic world, characters like Mad Max could be involved. He might see the homunculi as either allies or threats in the harsh new world. In a crossover with a cyberpunk world, characters like Neo from 'The Matrix' could be interesting. The homunculi could be part of the complex digital and physical world that Neo navigates, and he could help them understand the true nature of their existence in this new, high - tech world.
Hermione Granger from 'Harry Potter' could be an interesting addition. She could use her magic knowledge to study the mummy. Maybe she could find a spell to control or at least understand the mummy better. It would be a really unique combination of magic and ancient Egyptian mystery.
In a crossover with 'Fullmetal Alchemist', Edward Elric could be an interesting addition. His scientific knowledge and his search for the Philosopher's Stone could interact with the strange alchemy - like aspects of Rea's resurrection in Sankarea. He might be both fascinated and appalled by what has happened to Rea.
If it was a crossover with The Evil Within, Sebastian Castellanos would be a great character to include. His detective skills and his ability to handle the nightmarish creatures in his own game could be useful in the Outlast 2 world. He could team up with the Outlast 2 character to solve the mysteries behind the horror. Also, Pyramid Head from Silent Hill could be a terrifying addition. His presence alone would heighten the sense of dread in the Outlast 2 crossover story.
Batman would be a great character to include. His no - kill rule contrasts with Deadshot's assassin nature. Their interactions would be full of tension and moral debates. Harley Quinn could also be included. Her chaotic personality could add an element of unpredictability to the story. She might team up with Deadshot for a mission or be a distraction during one of his hits.
If it's a Moana and Hercules crossover, Hercules would be a great addition. His super strength could help Moana in situations where physical power is needed, like moving large obstacles in her path. Also, Philoctetes could be there to train Moana in some combat skills, adding an interesting dynamic to the story.
In a crossover with the Lord of the Rings, Aragorn could be an interesting choice. He could bring his leadership skills and combat expertise to Los Santos. The GTA 5 characters might initially be skeptical of his noble ways, but they could learn from him. He could also help them fight against a new evil force that has emerged in the city, perhaps an ancient and powerful gang with dark magic - like abilities.
Crash Bandicoot would be a great addition. His goofy and fearless nature would contrast well with Spyro's more noble and magical persona. He could bring some comical moments to the story while also being a strong ally in battles.