Here's a quick au idea that just came to me today:
In a modern au where Merlin is still waiting on Arthur, historians recently discovered a whole batch of legal documents from Arthur's reign. These documents detail many changes he made to the laws of Camelot, including the repeal of the magic ban (which the historians assumed was just a halt on witch hunts).
However, one thing that the historians note as strange were the large number of laws that only applied to the king's personal manservant, who was never mentioned by name in the documents. These laws range from oddly specific, such as 'the king's manservant shall not accompany knights to the tavern', to downright bizarre decrees that make no sense, like 'the king's manservant is hereby forbidden from pointing out stew in the king's hair.'
The historians' first guess was that perhaps King Arthur was going a bit mad in his later years, but they didn't find any other ludicrous laws besides the ones pertaining to his manservant, which then led the historians to question the identity of this manservant and his relationship to the king.
All of this culminates in a historical exhibit showcasing the documents and postulating on this mysterious manservant of king Arthur. Many scholars flock to the exhibit, eager to examine the documents and debate their meaning and impact within a historical context.
Which then leads to a very tired Dr. Merlin Emrys, a medieval history professor, being dragged by his colleagues to see the exhibit and having to stifle is laughter as these world-renowned scholars tear their hair out trying to understand what was essentially a prank war between him and Arthur.
