In many fanfictions, the scenario of Naruto and Itachi killing the Uchiha clan is a very interesting but also controversial topic. One aspect could be the change in Naruto's character. Usually, Naruto stands for justice and protecting others. If he were involved in such a massacre, it would be a huge deviation from his original character. Maybe in the fanfiction, there could be some extreme situations that force him to do so, like being mind - controlled or having some false information.
In some fanfictions, a possible motivation for Naruto could be to prevent a greater evil. Maybe he was tricked into believing that the Uchiha clan was about to start a war that would destroy the entire ninja world. So, he thought that by taking part in killing the clan, he could save more lives in the long run.
Well, in some fanfictions, Naruto might start by finding suitable partners within the village. Since he is the last of his clan, he could look for women who are willing to carry on the Uzumaki bloodline. He may also use his influence as a hero to encourage others to support his clan - revival efforts, like getting the village to provide resources for the new Uzumaki generation.
A reason could be that Itachi is framed as a traitor to the Leaf Village in the fanfiction. Naruto, being the defender of the Leaf, would then see Itachi as an enemy and take action to kill him to uphold the village's safety and honor.
One challenge is the lack of pure - blood Uzumaki. Since he is the last one he knows of, finding suitable mates with enough Uzumaki blood could be difficult. Also, the Uzumaki clan has a lot of enemies. Those enemies might try to stop Naruto from reviving the clan so they don't become a threat again.
In some fanfictions, Naruto might restore his clan by using a powerful jutsu to bring back the dead members. For example, a fictional super - level resurrection jutsu could be discovered. However, this would likely have many consequences and moral implications within the Naruto universe.