Harley Quinn is also a great choice. Her loyalty to the Joker and her own brand of insanity can make for some really fun interactions. If it's a crossover with other DC heroes, characters like Green Arrow could be included. He has a similar vigilante style to Batman and could bring in his own set of skills and a different perspective on crime - fighting.
Oracle, Barbara Gordon, would be an important character. She could serve as the information hub, coordinating between different groups of heroes in the crossover. And for a more out - there choice, Ra's al Ghul. His long - standing feud with Batman and his global influence could introduce some complex and far - reaching storylines when combined with other characters from different adventures.
Joker is a must - include. His chaotic nature would add a lot of drama to any crossover. He could be causing mayhem that draws in characters from other universes or teams. Another one is Catwoman. Her ambiguous relationship with Batman can create interesting dynamics, especially when other characters are involved.
Alfred Pennyworth could be included. He could offer his wisdom and support to both heroes. Maybe he even has some insights about the similarities and differences between Gotham and Starling City. Another character could be Felicity Smoak. She could help with the tech - related aspects of the case they are working on together.
Well, if it's a crossover with Ultraman, Ultraman could be a great addition. His super strength and light - based powers would contrast well with the Jetman's abilities. They could have a friendly rivalry at first but then team up to face a powerful kaiju that is threatening both their worlds.
In a crossover with 'The Last of Us', Joel and Ellie would be great additions. Joel's survival skills and Ellie's immunity could bring new dynamics to the State of Decay world. Also, from 'Left 4 Dead', the four survivors like Bill, Francis, Zoey, and Louis could be included. Their combat skills against zombies could be interesting when combined with the State of Decay gameplay.
Characters from 'Teen Titans' could be great. Their superpowers and the Saturdays' knowledge of the supernatural could lead to some amazing adventures. Imagine Raven's magic and Beast Boy's shape - shifting working together with the Saturdays. Also, characters from 'Gargoyles' could be interesting. The Gargoyles' nocturnal nature and combat skills along with the Saturdays' would create a really unique dynamic in the fanfiction.
Ash Ketchum is a must - include. His experience and bond with Pokémon can be a great starting point for any crossover. He could bring his Pokémon knowledge to a different world or interact with new characters.
Definitely Zeref. Since he has a connection to the Etherious and also to Natsu in the original Fairy Tail story. Another good character could be Gray Fullbuster. His ice - magic could contrast well with the more chaotic and dark powers of the Etherious. Also, Mavis Vermillion could be included. Her wisdom and powerful magic could play a role in helping Natsu deal with the Etherious side of things.
Well, you could include characters from other Power Rangers series like Tommy Oliver. He has a lot of experience and different powers from various ranger forms. His presence would add a lot of depth and an interesting dynamic to the story. Also, Bulk and Skull could be added for some comic relief. They could accidentally get involved in the Jungle Fury Rangers' mission and cause some chaos in their bumbling way.
One idea could be a crossover with another DC hero like Superman. Batman and Superman could team up to face a new, powerful villain that neither could defeat alone. Maybe this villain has the ability to manipulate Gotham's underworld and also poses a threat to Metropolis.
Well, in a Naruto - One Piece crossover, characters like Luffy and Naruto would be a must. Zoro could learn some sword - based jutsu from Naruto. And Nami might be interested in the ninja tools Naruto uses for stealth and thievery prevention on the ship.
In 'Batman Beyond Crossover Fanfiction', Terry McGinnis is a must - have. He is the main protagonist. Another popular character to include is Max, Terry's friend. She can provide support and a different perspective. Also, characters from other DC Animated Universes like Static Shock could be included. His electrical powers and street - smart attitude would contrast well with Batman Beyond's high - tech and more serious approach.