A great concept for a Super Saiyan crossover fanfiction would be to cross it with the Harry Potter world. Imagine a Super Saiyan wizard or witch. They could use magic spells while in their Super Saiyan form. Or perhaps a crossover with the God of War series, where the Super Saiyans fight alongside Kratos against the gods of that universe.
Well, you could do a time - travel based crossover fanfiction. Have a legendary super saiyan like Vegeta travel back in time to a different era in the Dragon Ball Z universe. He could meet his younger self and together they could face a new threat that was previously unknown. Or, a crossover with a video game world. Picture the legendary super saiyan Trunks entering the world of Final Fantasy and having to go on a quest there.
For a 'godlike saiyan crossover fanfiction', a crossover with the Marvel Universe would be exciting. Picture a godlike Saiyan arriving on Earth during the events of 'Avengers: Endgame'. They could help the Avengers in their fight against Thanos. Their super - strength and energy - blasts could turn the tide of the battle. Or perhaps a crossover with the 'Lord of the Rings' world. The Saiyan could be a powerful ally in the fight against Sauron, using their speed and power to take on the orcs and other dark creatures.
Another interesting concept is to cross them over with a fantasy world. Saiyans could be in a world full of magic and mythical creatures. Picture Vegeta having to deal with dragons that can talk and cast spells. Their Saiyan pride might get in the way at first when they realize that their brute strength isn't always enough in this new world.
Another concept could be a Saiyan - Naruto hybrid. A character who has both the natural talent for ninjutsu like Naruto and the raw power of a Saiyan. This character could struggle with balancing the two different power sources and cultures. They might face discrimination from both the Saiyan and ninja communities until they prove themselves as a unique and powerful force.
Well, a common plot in Goku Super Saiyan God crossover fanfiction is the fusion concept. For example, Goku could fuse with a powerful character from another universe. Let's say he fuses with Saitama from One Punch Man. This new fused character would have an insane amount of power. Also, a time - travel plot where Goku goes to a different era in another fictional world. He could end up in the future of the Terminator world and have to battle Skynet's machines using his Super Saiyan God powers.
Another plot could involve a Saiyan being sent to the Naruto world by accident. This Saiyan could initially be seen as a threat but then team up with Naruto and his friends to fight a common enemy. They could teach each other their unique skills. For example, the Saiyan could show Naruto how to fight without relying so much on ninjutsu at times, and Naruto could teach the Saiyan about chakra control.
Well, perhaps the Super Saiyan 4 semblance gives the character the ability to emit a powerful energy aura. This aura could have different effects, like being able to disrupt the Grimm's dark energy or even being used as a shield against attacks. In a fight, the character could expand the aura to push back groups of Grimm or focus it into a beam - like Goku's Kamehameha - to take out a strong enemy. It would be a very flashy and powerful semblance in the RWBY universe.
One popular example could be a crossover with Dragon Ball and Naruto. In such a fanfiction, a Super Saiyan 4 character might enter the Naruto universe and have to face off against powerful ninja. Another example could be a crossover with One Piece. The Super Saiyan 4's incredible power would be a huge contrast to the Devil Fruit users in One Piece. There are also crossovers with DC Comics, where the Super Saiyan 4 could potentially battle Superman or other powerful heroes.
One idea could be Ash getting transported to the Dragon Ball universe and somehow obtaining the power of Super Saiyan Blue. He could then use his Pokémon training skills combined with this new power to fight off powerful villains in the Dragon Ball world.
One idea could be a story where the Saiyan Blue form has an unexpected side effect. For example, it could start to drain the user's life force slowly but surely. Goku discovers this during a battle and has to find a way to either control it or reverse the effect while still fighting powerful foes.