It could be about a minor issue that Merlin faces in Camelot. Maybe it's a small spell - casting mishap that causes some comical chaos in the court. For example, he accidentally turns Arthur's hair pink while trying to do a simple cleaning spell on his chambers.
Another possibility is that it's about a problem within the magical community that Merlin is a part of. There might be a small rift among the sorcerers due to a new, minor magical law that Merlin has to deal with. He might be caught in the middle of this as he tries to keep the peace between the different factions while still hiding his true identity from most of Camelot.
First, focus on the character of Merlin. Since it's a 'tiny problem', it should be something that doesn't overshadow his main role but still adds depth. For example, you could start with Merlin waking up one day and realizing he has lost his ability to communicate with his familiar, which is a small but significant part of his magic.
Perhaps a story where Tiny Merlin accidentally shrinks the whole Camelot and has to figure out how to reverse the spell. He would have to use his wits and small - size advantages to solve the problem. This could involve sneaking into small spaces in the shrunken Camelot to find the source of the spell. It would be a really fun and adventurous story.
Obviously, Merlin himself. And Arthur could be in it too. Maybe Arthur doesn't initially recognize Tiny Merlin and there are some comical misunderstandings between them.
It could also be that in a superhero fanfiction, the superhero has a weighty problem such as having to choose between saving a large number of people in a city or preventing a global catastrophe that requires their immediate attention elsewhere.
One possible reason could be a magical curse. In many Merlin fanfictions, magic is often used as a plot device. A dark sorcerer might cast a spell on Merlin to make him forget certain things, perhaps to prevent him from using his powerful magic to oppose them. Another reason could be a self - imposed forgetting due to trauma. Merlin has witnessed and endured a great deal of pain and suffering. To protect his own mental state, his mind might subconsciously block out certain memories.
Well, the plot could involve Naruto learning unique magic skills from Merlin. Maybe Merlin helps Naruto control his chakra in a new way using magic. For example, Naruto could learn to blend his shadow clone jutsu with illusion magic he gets from Merlin.
One possibility is that there would be a great deal of tension. Old Merlin has seen so much and has his own wisdom, while Morgana has her own powers and motives. Maybe Merlin would try to persuade Morgana to abandon her dark path. He could use his experience and the bond they once had to appeal to her better nature.
Well, perhaps in the fanfiction, Harry Potter might find a sense of peace in the tiny house. It could be a place where he can hide from the chaos of the wizarding world. He might decorate it with magical trinkets he's collected over the years, and it could be a place where he studies new spells in solitude.
The plot could involve Harry discovering his powerful heritage. Maybe he has to deal with the fact that his parents are two such significant and complex figures. He might face challenges from those who fear his potential power due to his lineage. He could also embark on a journey to learn more about Merlin and Morgana's history and how it relates to him.
The fanfiction could delve into the emotional turmoil that such a kiss would cause. Think about it. Merlin has his loyalty to Arthur and Camelot, and Morgana has her own path of revenge or power - seeking. This kiss could be a moment of weakness or a sign of a deeper, hidden love that throws all their plans and allegiances into chaos.