One interesting plot could be Merlin hiding his pain from Arthur because he doesn't want to seem weak. He might endure the taunts of the knights in silence while secretly using his magic to heal his internal wounds. Another could be Merlin hiding his pain over his mother's absence or the fact that he has such a huge responsibility on his shoulders with his destiny to protect Arthur and Camelot.
The main character is Merlin, of course. He is the focus as he is the one with the hidden pain. Arthur Pendragon is a key figure too. Since Merlin is Arthur's manservant, Arthur's actions, words, or even his ignorance of Merlin's true self can be sources of Merlin's pain. Sometimes, the Great Dragon can also be involved. The dragon might be aware of Merlin's pain and offer some cryptic advice that Merlin has to decipher while still hiding his pain from others.
One possible plot could be that Hiccup hides his strength to protect his village in a more unexpected way. For example, he pretends to be weak while secretly training a special group of dragons to be a surprise defense force.
Well, one possible plot could be that Harry hides his power to protect his friends from dark wizards who might target them if they knew how powerful he really was. Another could be that he hides his power to secretly investigate a mystery within Hogwarts without drawing attention.
Well, sometimes in these fanfictions, Draco hides his pregnancy because he's in a secret relationship. For example, if he's in love with someone from a different house or a different social standing. He might be afraid that if his pregnancy is discovered, the relationship will be exposed and there will be consequences. Also, there could be a plot where he discovers some magical mystery related to his pregnancy that makes him think he has to hide it, like there's a prophecy or some dark magic involved.
Another plot could be that Merlin's brother is initially on the wrong side. Maybe he's been influenced by Morgana or some other dark force. But through Merlin's love and persuasion, he eventually sees the error of his ways and joins Merlin in the fight for good.
A plot might involve Merlin leaving Camelot to seek greater magic knowledge. He has learned a lot in Camelot, but there are still secrets about his powers and the old magic that he can't uncover there. So, he sets off into the unknown, leaving behind his friends and the life he has known. This could lead to him encountering new magical creatures and learning ancient spells that he can later use to help Camelot in a time of great need.
One interesting plot could be Merlin's discovery of unique magical powers that are a result of being raised by dragons. For example, he might have an innate ability to communicate with all magical creatures, not just dragons. Another could be his struggle to fit into the human world while having such a different upbringing. He may face challenges in understanding human customs and politics.
A plot that often shows up is the idea of soulmates. In fanfiction, Merlin and Morgana could be fated to be together as soulmates. Their love is written as something that is destined, and no matter how much the world tries to keep them apart, they keep being drawn back to each other. Another interesting plot is that Morgana has a premonition of their love and future together, and this changes her attitude towards Merlin from the start. She then tries to find ways to make their relationship work despite all the odds in Camelot. And there are also fanfictions where they meet in secret and have a secret relationship, with all the tension and excitement that comes with hiding their love from the people around them.
Another possible plot is that Hermione, as the heir of Merlin, is prophesied to bring about a new era of magic. However, there are those in the wizarding world who oppose this change. So she has to gather allies, including some unexpected ones, and fight against the old - fashioned and conservative forces that want to keep the status quo. Along the way, she also has to deal with her studies at Hogwarts and her relationships with her friends like Harry and Ron.
One interesting plot could be Naruto delving deeper into the understanding of pain not just physically but also the emotional pain of his enemies. This might lead to him trying to find more peaceful solutions in battles.