It could be a result of some ancient jutsu that was passed down but lost in the main Naruto story. Maybe it was a power given to him by a mysterious entity to help him in his journey.
It could be from some sort of ancient ninja training that was lost and he somehow discovers it. Maybe there's a hidden scroll or a hermit who teaches him this fire - controlling ability.
One possible origin could be some kind of ancient bloodline. Maybe Naruto has a hidden bloodline that was passed down through generations and finally awakened, allowing him to summon angels. Another option is that he got this ability through a special training or a quest. He might have found a hidden place or an old sage who taught him the art of summoning angels.
One possibility is that he inherits it through some sort of bloodline connection that was previously unknown. Maybe there's a distant relative who had the ability to control sand, and it somehow got passed down to Naruto.
One possibility could be some sort of new training he undergoes. Maybe he discovers a hidden jutsu scroll that teaches him how to control and produce fire from within his body. This could be a long - lost technique from an ancient ninja clan.
Well, in some fanfictions, Naruto's water - controlling ability could be a gift from a powerful sage. This sage, seeing Naruto's potential and his need for new powers to face upcoming threats, bestows upon him the ability to control water. Another origin could be that Naruto accidentally absorbs the chakra of a water - using enemy during a battle, and this chakra somehow melds with his own, giving him the power to control water.
One possibility is that it could be related to some ancient clan's hidden jutsu that Naruto somehow discovers. Maybe it was lost for a long time and Naruto stumbles upon it during one of his adventures.
Perhaps it's related to his connection with the Nine - Tailed Fox. In the original Naruto story, he has a strong bond with the Nine - Tailed Fox. In the fanfic, this bond could have evolved in a way that allows him to summon foxes. It could be a manifestation of the latent power within him that he finally manages to tap into and use to summon these fox - like creatures.
It might be related to an ancient magic that she unknowingly tapped into. There could be some long - forgotten spell or charm that she came across in her extensive research. It could also be a gift from a magical creature. Maybe she helped a magical snake in the past, and in return, it passed on the ability to communicate with snakes to her.
It could be a result of some kind of rare genetic mutation. Maybe there was a dormant gene in his Uzumaki bloodline that got activated under certain circumstances, like during a near - death experience or a powerful chakra surge.
In some fanfictions, it could be due to a genetic mutation or an unexpected side - effect of a jutsu he uses. For example, if he experiments with a new form of chakra manipulation, it might accidentally give him the power to summon swords. Or maybe it's a gift from the Sage of Six Paths in a different interpretation of the Naruto universe.