I just started watching fire force and it's pretty cool. As I'm on ep4, I know very little about the plot, so I won't spoil much.
I think having an Uchiha with the ability to use only fire type ninjutsu would be pretty cool. They obviously have the sharingan which could evolve to 3 tomoe's naturally, and the mangekyo when the 'strange people' involving the main character (of fire force), kill his family.
Keeping the handsigns is a must as that raises the coolness factor, and it serves as a nerf/buffing point without people realizing it.
Sharingan abilities should also conform to both Naruto and Fire Force rules (at least on the surface).
For basic sharingan abilities improved dynamic vision is a must. In fact, illusion abilities could be non-existent and dynamic vision abilities and increased reflexes could be capitalized on. If illusions are used, flames and themes from Fire Force should be used. For example, burning people suffering, flames watching them instead on crows.
The warning here though is that the main character shouldn't excessively use the sharingan though. First, it's bad to completely rely on the sharingan. Secondly, it seems way cooler (imo) if only used when confronting people with ulterior motives, dangerous enemies, or tragic incidents.
Capitalize on the fact that the sharingan forces members on the Uchiha clan to always be able to remember tragedies they see when using the sharingan.
For mangekyo, Susanoo could be the same, but flaming. The second ability could be something like Madaras time reversal or lean more into Fire Force theme like Amaterasu. Just try not be make it stupidly obvious if it doesn't conform to Fire Force's rules.
I also think that the theme should be kept about losing 'light' for the eyes. This would be a crucial reason to not constantly use mangekyo, a reasonable nerf. This also adds a lot of room for character development.
This is a side rant, but I think that most people think that they need to overly explain for character development. But I like the small emotional cues to signal character development.
For example, making more rational decisions over emotional decisions. For an Uchiha especially, this shows character development.
I didn't explain this idea of subtle character development well, if you read some Chinese novels, you might understand, they use that all the time.
It would be cool if someone wrote this or something similar.