A really cool plotline could be a modern reincarnation story. Arthur and Merlin are reborn in the modern world and start to remember their past lives. They have to come to terms with their old relationship and also face new challenges in the modern world, like dealing with modern technology while still having their ancient magical bond.
One interesting plotline could be Merlin using his magic in modern times to secretly protect Arthur, who is a powerful businessman or politician. For example, he might use magic to prevent assassination attempts on Arthur.
One interesting plotline could be the knights and Arthur facing a new, powerful dark magic threat that only Merlin's unique powers can help them overcome. They might journey to a far - off land in search of a magical artifact Merlin has foreseen as the key to victory.
One interesting plotline could be Arthur's initial shock and then his gradual acceptance of Merlin's new identity. He might start to see Merlin in a whole new light and realize how much power Merlin truly holds.
One interesting plotline could be their journey to discover their true family history. Since they are brothers in this fanfiction, there might be secrets about their parents or heritage that they uncover together. Another could be them facing challenges as brothers, like one getting into trouble and the other having to save him. For example, Arthur might get captured and Merlin uses his magic to rescue him, strengthening their bond as brothers in the process.
One common plotline is Merlin using his youth to his advantage. For example, he might be able to blend in more easily with the younger generation in the kingdom, gathering information that Arthur can't. Another is Arthur having to deal with his own insecurities about getting older while Merlin is full of energy. Also, there could be a plotline where Merlin's younger age makes him more adaptable to new magic or technologies that are emerging, and Arthur has to rely on him for that knowledge.
One common plotline could be Arthur realizing his roughness is wrong and trying to redeem himself. Another might be Merlin getting fed up and leaving, which then makes Arthur see how much he needs Merlin.
One common plotline is Merlin using his age - related wisdom to save Arthur from various threats. For instance, he might have prior knowledge of a magical curse that's about to befall Arthur and uses his experience to counter it.
One interesting Merlin and Arthur fanfiction could be where Merlin reveals his magic to Arthur in a very unexpected and comical way. For example, during a royal banquet, when a knight accidentally drops a sword that's about to impale Arthur, Merlin freezes time and casually walks over to pick up the sword, then unfreezes time and acts like nothing happened. Arthur is left completely bewildered until Merlin finally confesses his powers.
Another plotline might involve Merlin coming back to find Camelot in chaos. His return then becomes a journey of restoring order, not just through his magic but also by reuniting the people and rebuilding their trust in the kingdom. He might have to deal with those who have misused power in his absence.
One interesting 'arthur forgets merlin fanfiction' could be where Merlin has to use his magic in new and secretive ways to re - acquaint Arthur with their friendship. Maybe he creates small magical incidents that jog Arthur's memory in a subconscious way.
There might be a fanfiction where Arthur and Guinevere face a political crisis in Camelot. Merlin, with his unique insights, helps them navigate through it. He might use his magic subtly to influence the nobles in favor of the king and queen's decisions. For example, he could make a symbol appear during a council meeting that sways the nobles to support Arthur and Guinevere's new policy for trade with a neighboring kingdom.