Plot: Naruto is on a mission to Iwa. While on the way there he finds a cave inside a mountain. He goes inside the cave and all of a sudden the floor collapses. When he wakes up he finds out that he is in the Village Hidden in the (you can decide of a name for it, but try to make it original). After talking to Kyubi he decides to stay in the village.(because Kyubi threatens him that if he leaves the village then he will cause Naruto great pain). While he living underground he will face new problems, new people, and maybe find love, but up above his friends are wondering where he is, or if he had died. Also the Akatsuki is still on the loose and looking for him too. The Kyubi also has a secret that Naruto doesn't know. It is why he wanted to stay in the village.
Pairings: Naruto/OC(someone from the underground village) and then whatever else you want to do.
This takes place before Sasuke defects. Naruto doesn't know that Sasuke defects. Tsunade sent Naruto on a mission to Iwa before Sasuke could get a chance to ask Naruto to fight him.
If you want to write this or have any questions then please pm me.