One storyline could be that the British mage is accidentally transported to the Naruto world. They then have to find a way to get back home while also dealing with the various threats in the Naruto universe. Along the way, they could make friends with Naruto and his friends and use their magic to help in battles against the Akatsuki.
There could be a storyline where the British mage is a fugitive in their own world and flees to the Naruto world to escape persecution. In the Naruto world, they try to keep a low profile but are eventually discovered. Naruto and his friends, instead of turning the mage in, offer to help them fight against those who are after them. In return, the mage uses their magic to enhance the strength of the ninja village they are in.
A British mage might struggle to fit in at first. In the Naruto world, power is mainly based on chakra. But the mage could start by using their magic secretly. For example, they could use magic to heal wounds in a way that's different from medical ninjutsu. Eventually, they might teach some of their magic to Naruto characters or learn from them about chakra. It could be a two - way exchange of knowledge and skills that would change the Naruto universe in unique ways.
One possible storyline could be Naruto joining Fairy Tail and having to prove himself to the guild. He'd use his wind mage powers to take on quests and fight against dark guilds. Another could be him facing off against a powerful wind - based enemy in the Fairy Tail world and having to learn new techniques to defeat them.
One unique storyline could be a forest mage Naruto who discovers a hidden grove in the forest that holds ancient chakra secrets. He then uses this knowledge to develop new jutsu that are nature - based and can heal the land as well as fight enemies. Another could be a forest mage Naruto who forms a bond with forest spirits and they help him in battles, granting him special powers like camouflage or enhanced speed among the trees. And there could also be a story where Naruto, as a forest mage, has to protect the forest from dark forces that want to destroy it for its powerful natural energy.
His chakra. Chakra is like the energy source in Naruto's world. In the fanfiction, this chakra could be translated into a very powerful form of magic in Fairy Tail. It gives him the ability to perform various powerful techniques, much like mages in Fairy Tail use their magic.
In that fanfiction, it might be designed that way. Naruto has incredible powers in his own series, like his various jutsu. If translated into the Fairy Tail world in fanfiction, his skills could potentially make him very powerful. But it also depends on how the author of the fanfiction interprets and balances the power systems of both universes.
The unique feature is the blend of magic and ninjutsu. For example, in some fanfictions, characters can cast spells like mages while still using their ninja tools. It's like a new power - up for the Naruto characters.
One possible storyline could be a crossover where Naruto and M'gann are brought together in a strange new world. Amaterasu could be the key to getting them back to their respective universes. For example, they might have to find the origin of Amaterasu's power in this new world to unlock a portal. Another storyline could be that M'gann is captured by an enemy who wants to study Amaterasu's power, and Naruto has to rescue her.
In 'naruto is british fanfiction', one idea could be that the chakra system is related to some sort of British magic. Naruto might discover that his ninja powers have a connection to ancient British magic that was long forgotten. He could then go on a journey to uncover the secrets of this connection, meeting British - inspired characters along the way who either help or hinder him in his quest.
In this fanfiction, Kakashi could also be Naruto's mentor in magecraft. Kakashi is known for his adaptability. He could start to study mage powers along with Naruto. They could learn together about new forms of magic, and Kakashi could use his combat experience to help Naruto understand how to use his mage powers in battle situations.
Power and its consequences. Since black mages often have great and sometimes dangerous powers, many fanfictions explore how this power affects the character and the Naruto world around them.