One interesting Fate Merlin fanfiction could be about Merlin guiding a young and inexperienced mage in the modern Fate universe. This could involve him using his ancient magic knowledge to help the new mage face challenges.
One interesting fanfiction could be about Merlin guiding a different Servant through the Holy Grail War in ways not shown in the original. For example, he might use his magic to give a less powerful Servant an edge in battle by revealing hidden strategies of the opponents.
Perhaps there's a plotline where Merlin discovers that his bond with Arthur is not just a matter of loyalty and friendship, but is actually a magical bond that was created by an ancient magic long before they were born. And as they face various challenges, this bond either strengthens or is put at risk depending on their actions.
A great Merlin stories fanfiction idea is one where Merlin has to go back in time to correct something that went wrong in Camelot's history. This could involve him having to face his younger self and convince his past self to make different choices, all while trying not to disrupt the timeline too much.
There are some that depict Merlin as a street magician in modern London. He uses his real magic in his shows, but people think it's just clever illusions. He meets Arthur, who is a skeptic at first but then becomes intrigued by Merlin's seemingly unexplainable tricks.
One interesting proactive Merlin fanfiction could be where Merlin uses his magic more openly to change the political landscape in Camelot. For example, he might secretly influence laws that are unfair to the common people. Another could be about Merlin taking proactive steps to train Arthur in magic, defying the ban in Camelot. This would add a whole new dynamic to their relationship as Merlin tries to make Arthur see the value of magic in a different way.
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.
In a footloose Merlin fanfiction, perhaps Merlin and Arthur are both in a land where dancing is forbidden. Merlin, being the more rebellious of the two in this regard, decides to secretly teach Arthur some dance moves. As they practice in a hidden corner of the castle, they form a closer bond, and Merlin uses his magic to make sure they don't get caught. This could be a fun and light - hearted fanfiction exploring their relationship in a new context.
Well, in some Merlin death fanfiction, Merlin might die in a battle against an ancient evil. His death could lead to a series of events where Camelot starts to crumble without his magic and wisdom. However, his legacy lives on through the people he has inspired. There are also stories where Merlin's death is foretold in a prophecy, and he spends his last days trying to prepare Camelot for the future without him.
There could be a fanfiction where Merlin uses his magic in a different way than in the original series. For example, he might use it to create a magical school in Camelot, teaching other young magic - users how to control their powers. This would change the whole dynamic of Camelot and its relationship with magic.
One interesting Merlin is king fanfiction could be about how Merlin uses his magic to bring about a new era of peace and prosperity in Camelot under his rule. He might introduce new laws that are fair to all, regardless of their social status or magical abilities.