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.
Definitely include Zak Saturday. He has his unique connection to the Cryptids and his own set of skills. Also, Raven from 'Teen Titans' would be a great addition. Her dark powers and mysterious nature would contrast well with the more action - based world of the Secret Saturdays.
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 addition to the Saturdays family, there could be other recurring characters in different fanfictions. For example, some might introduce a new young cryptid hunter who teams up with Zak. This character could have a different set of skills and a backstory that adds more depth to the story. Maybe they come from a long line of hunters but have a more modern approach compared to the Saturdays. However, the core always remains the Saturdays family as they are the central part of the original concept.
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.
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.
Obviously, Sakura herself is a must - include. Then, characters like Kakashi could be involved as he has Anbu experience. Naruto and Sasuke might also make appearances, maybe Naruto trying to help Sakura in her Anbu - related adventures, and Sasuke being a bit of a wildcard. Additionally, some original Anbu characters could be created, like a veteran Anbu agent who becomes Sakura's mentor in the Anbu world.
One idea could be a crossover with 'Ben 10'. The Secret Saturdays could team up with Ben to fight a common enemy that is threatening both their worlds. Maybe an ancient alien force that wants to take over Earth and has knowledge of both the Cryptids and the Omnitrix.
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.
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.
Sasuke can also be a popular character. His brooding nature and powerful Sharingan can add a lot of depth to the fanfiction. In a Naruto Secret Saturdays crossover, he might be intrigued by the different powers and creatures in the Secret Saturdays world and try to gain more power from them.