I'm not sure about a specific one that stands out above all others. But if you search on major fanfiction platforms using the keywords'merlin', 'cat ears', you're likely to find a bunch. You can also look at the reviews and ratings to find the ones that are well - liked. Some fics might also include other magical creatures or elements related to Merlin's cat ears, like how they might give him special powers in certain situations.
In Merlin fanfic, 'a cat may look at a king' could imply a sense of equality in an unequal world. It might show that even a seemingly low - status character (the cat, representing Merlin perhaps) has the right to interact with or observe a high - status character (the king, like Arthur).
Another possibility is that the cat is actually a transformed being. It could be a person who was cursed and Merlin has to find a way to break the curse. The cat could communicate with Merlin in a special way, like through dreams or by leaving paw prints that form messages.
Another interesting concept is the social aspect. How do the people of Camelot react to Merlin having cat ears? Some fics show that the common folk might be superstitious and think he's a bad omen, while others in the court might see it as a novelty. It makes for some really complex character interactions and plot developments. For example, Morgana might use this to her advantage to try and turn the court against Merlin, or she could be sympathetic and help him hide it. And of course, there's Arthur's reaction, which can range from teasing to protective depending on the writer's take.
Well, you can start by looking on fanfiction.net. There are often many Merlin fanfics there that might include the concept of 'a cat may look at a king'. Just use their search function and type in relevant keywords.
The cat could have a special bond with Arthur too. Maybe it likes to curl up on Arthur's lap when he's in a pensive mood, and this makes Arthur more open - minded about magic. The cat could also be involved in a situation where Arthur has to rely on Merlin's magic, which the cat is somehow connected to, to save his life.
One great female Merlin fanfic could be one where she uses her magic in unique ways to save Camelot. For example, in a story she might use her powers to heal the knights secretly when they are injured in battle. This would add an interesting twist as she has to hide her actions from Uther.