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.
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.
Well, he could start a family with Hinata and have lots of kids, passing on his Uzumaki bloodline. Then, he could encourage his friends who have some Uzumaki connection, like Karin, to also have families and pass on their Uzumaki genes. He could also set up a school or training center in the Uzumaki land, teaching the Uzumaki ways of life and ninjutsu, which would attract orphans or those who want to be part of the Uzumaki clan.
One challenge could be getting the other ninja clans to accept his new clan. The existing clans have their own traditions and power structures, and they might see Naruto's new clan as a threat.
In some fanfictions, Naruto might use a powerful jutsu he discovers or masters. For example, he could find a forbidden jutsu that has the power to bring the dead back to life. Maybe it's a jutsu that requires a great sacrifice on his part, like giving up some of his own chakra permanently.
One challenge would be getting other shinobi to join. Some might be loyal to their existing clans. Also, there could be political opposition in Konoha. The elders might not be too keen on a new clan being formed.
In harem fanfiction, Naruto could face the challenge of dealing with different cultural backgrounds of his partners. Since they come from various places, integrating their ideas and values into the new clan structure might be a headache. Moreover, resources are always an issue. Rebuilding a clan requires a lot of money, manpower, and materials. He has to figure out how to gather these resources while also maintaining his relationships in the harem. For example, some of his partners may have their own financial or family obligations that could either help or hinder his clan - rebuilding efforts.
In some fanfictions, Naruto might use a powerful jutsu. Maybe he discovers an ancient sealing technique that can bring back the dead for a short time. He could also have found some rare chakra-infused artifact that has the power to revive the First Hokage.
In certain fanfic concepts, Naruto could have found a special relic or artifact. This relic might have the power to reverse death. Since Naruto is always on adventures and exploring different places, he might stumble upon such an item and use it to revive Kushina. It could also be related to the Uzumaki clan's secrets that he uncovers which enables him to bring his mother back.
In some fanfictions, Naruto might use his strong influence and the power of his bonds. He could convince the other villages to help, pooling resources like building materials and ninja manpower. With his own Uzumaki heritage and knowledge of Uzushiogakure's past, he'd guide the reconstruction process.
In fanfiction, Naruto may face challenges related to politics. Other villages may see the restoration of the Uzumaki clan as a threat to the balance of power. They could try to interfere in Naruto's efforts through various means such as spreading false rumors about the Uzumaki or even sending spies or assassins. Additionally, Naruto has his own duties as Hokage, so he would need to balance his time between running the village and restoring his clan.