Another idea is a crossover with a superhero universe. Imagine a character like Sasuke with his Mangekyou Sharingan powers suddenly in the Marvel or DC universe. He could team up with heroes like Spider - Man or Batman. His Sharingan's ability to copy moves could be really interesting when facing off against super - villains.
One possibility could be a story where the power of the Sharingan from Naruto is introduced into the Avatar world where Azula exists. Maybe a character with Sharingan ends up in the Fire Nation and has to face off against Azula. This could lead to some really interesting battles and power dynamics as the Sharingan's abilities like seeing chakra flow and predicting movements would be new and challenging for Azula and her firebending skills.
Another theme is the clash of cultures. The sharingan comes from the Uchiha clan which has its own traditions and values. When combined with the Naruto universe which has its own diverse set of cultures, it leads to interesting situations. For instance, a sharingan user might have to adapt to the way of life in Konoha which is different from where they originally come from in their own universe.
In a Sharingan Naruto Highschool DxD fanfiction, the Sharingan could also play a role in terms of illusions. Imagine a scenario where the character uses Sharingan - based illusions to deceive the enemies from both worlds. They could make a Highschool DxD devil believe they are in a completely different environment like a dangerous Naruto - style forest filled with traps. And for Naruto characters, they could be made to see illusions of their worst nightmares or false threats from the Highschool DxD world, which would make for some really intense and engaging storylines.
Sasuke is often very popular. His Sharingan abilities are quite powerful and his brooding personality makes for interesting crossovers. Writers can put him in different situations where he has to use his Sharingan in new ways.
One interesting concept could be a character from another universe getting the Sharingan. For example, a character from a modern - day setting suddenly having the power of the Sharingan. It would create a whole new dynamic as they learn to use it in a world without chakra.
One great 'Sharingan Naruto Crossover Fanfic' is 'The Uchiha in Konoha'. It has an interesting plot where an Uchiha character with the Sharingan has some unique adventures in the Naruto world. The author does a great job of blending the powers of the Sharingan with the existing Naruto lore.
One interesting storyline could be Naruto using the Sharingan to enhance his combat skills in a new and unexpected way. For example, he might combine his Rasengan with Sharingan - powered genjutsu to create a more powerful and deceptive attack.
A Naruto and Harry Potter crossover with the Sharingan involved would bring a whole new dynamic. The Sharingan's powers of perception and the ability to copy techniques could be adapted in the magical realm. The fanfiction could explore how a ninja's training and skills would fare against the magic of the wizarding world. There could be a lot of cultural clashes as well. The ninjas' values and ways of life are quite different from those in the wizarding world, and this could lead to some really engaging story arcs where they learn from each other and grow.
The character with the Mangekyou Sharingan might lose their eyesight due to overuse in the new and unfamiliar world of the crossover, but then gain a new form of power that is related to the crossover world. Maybe they lose their Sharingan sight but gain the ability to see magic auras or something similar. This would be a huge twist for both the character and the readers.