One idea could be a story where Obito's mask holds a hidden power. For example, it could be a key to an ancient jutsu that was sealed away long ago. Another plot could involve a character trying to steal the mask for its significance in the ninja world.
Obito's influence would likely make Naruto more aware of the darker side of the ninja world. Naruto is usually very optimistic, but with Obito's guidance, he would see the hardships and betrayals that can happen. For example, Naruto might become more cautious in his trust of others.
In the 'naruto has a mask fanfiction', it might be that Naruto finds the mask himself. He could stumble upon it during one of his adventures, perhaps in an ancient ruin or a hidden cave. The mask could be enchanted or have some special property that draws Naruto to it, and he decides to keep it and figure out its secrets on his own.
Besides Obito, Sasuke could be a main character in this fanfiction. Sasuke, with his own journey of redemption and his understanding of power and the ninja world, could have an interesting dynamic with the reincarnated Obito. Also, Sakura might play a role. She could be the one who tries to heal the reincarnated Obito, both physically if he is injured during his new adventures and emotionally from the traumas of his past life.
The mask in the 'naruto has a mask fanfiction' might be a symbol of mystery. It could hide Naruto's true identity or feelings. Maybe he uses it to conceal a new power he has acquired and doesn't want his enemies to know about right away.
His enemies, too. They might realize that Naruto is not as straightforward as they thought. If he shows a more cunning side by dropping the mask, they would have to adjust their battle strategies. For example, Orochimaru might find Naruto a more formidable opponent than he initially believed.
A major theme is the power of forgiveness. Naruto is all about forgiving others, and when it comes to Obito, this theme is often explored. How can Naruto forgive Obito for all the harm he has caused? Also, the theme of changing one's fate. Obito's fate seems set in the original story, but in fanfics, he often tries to change it with Naruto's influence. There are also themes related to the understanding of different ideologies as Naruto and Obito have very different outlooks at times.
In some fanfictions, it might be so. Fanfiction allows for creative and often out - of - canon interpretations. However, in the original Naruto series, Naruto has no relation to Obito in this way.
In fanfiction, the combination of Naruto, Temari and Obito can lead to some really interesting storylines. For example, they could be on a joint mission where their different skills and personalities clash and then gradually come to work well together. Naruto's determination, Temari's no - nonsense attitude and Obito's complex past could create a lot of drama and character development.