In 'under lock and key fanfiction', it may deal with power dynamics. For example, one character might have another character under their control, much like something being locked away and only accessible or manipulable by the one with the key. It could also be about a hidden society or group that is 'under lock and key', meaning it's very exclusive and secretive, and the fanfiction explores the inner workings of this group.
In 'under lock and key miraculous fanfiction', a frequent theme is struggle. The characters are likely to struggle against their confined situation. It could also involve themes of discovery. As they try to break free from being locked away, they might discover new things about themselves, their powers, or the Miraculous world in general. For example, they could find out about a long - lost power or a hidden enemy. And of course, there's the theme of trust. They may have to trust in new allies or their own instincts to get out of the locked - in situation.
I'm not sure about specific 'under lock and key miraculous fanfiction' off the top of my head. But you can try searching on fanfiction platforms like Archive of Our Own. They usually have a wide variety of Miraculous fanfictions, and you might be able to find ones related to 'under lock and key' there.
It could be about a character's heart being metaphorically locked away due to past traumas. Maybe a Fairy Tail member has faced a great loss and has closed off their emotions. The story might revolve around another character trying to unlock that heart, leading to a journey of self - discovery and friendship within the Fairy Tail guild.
It could be Natsu. He's a very popular character in Fairy Tail. Maybe he experiences something that makes him lock his heart away, like the fear of losing his friends. His journey to unlock it could be full of action and his typical fiery determination.
The stars are a key element. They often serve as a backdrop, creating a sense of wonder and vastness. Harmony could be achieved through the characters' interactions, like when they share a common goal while looking at the stars, such as finding a particular constellation.