Rather than sharingan, fire nature users are common in the Uchiha clan. Sasuke is shown to be using variations of Chidori a lot more than fire jutsu (pre-amaterasu), which is why he can be seen as a lightning user. As for Danzo, since he has shisui's eye, Tencent probably needed a ninja for the fire-lightning barrier (fire-wind-lightning when skill broken - just like shisui).

As for OP, danzo's affinity as fire makes sense compared to other ninjas where they don't have any nature chakras. For example, why are shikamaru and his dad are earth nature just like someone like shino, ino or enma? In the end, it doesn't matter in the game.