The origin of dragons in the context of the Lord of the Rings universe can be a theme. Some fanfics might delve into the creation of dragons, whether they were made by Morgoth or if there is some other backstory. There could also be themes related to the dragon's connection to magic. Since magic is an important aspect of Middle - earth, fanfics might explore how a dragon's magic is different from that of wizards or elves and how it affects the world around them.
One common theme is the power struggle. Dragons are often depicted as extremely powerful beings, so fanfictions may show them being either a threat to the established powers in Middle - earth or being courted by different sides for their power. Another theme is the mystery of dragons. Their origin, their true nature, and their connection to the magic of the world are often explored.
One common theme is the clash of different cultures. For example, when Middle - earth characters meet those from other fictional worlds, there are often misunderstandings at first due to different values and ways of life.
One common theme is self - discovery. Many original characters in the fanfiction are on a journey to find out who they are in the context of Middle - earth. For example, an OC might be a human with some elven blood and they are trying to understand both parts of their heritage.
One common theme is the clash of different powers. The high - tech powers of the Avengers against the magic in Middle - earth.
One common theme is the clash of different natures. Vampires, with their need for blood and immortality, are different from the races in Middle - earth. So, there's often a theme of how they are accepted or rejected. For example, elves might view them as abominations while humans might be more divided.
In 'Lord of the Rings' fanfic, apart from 'age play' which is not a common theme at all, many fanfics explore the magic of Middle - earth in more detail. They might describe the workings of Elvish magic or the power of the One Ring in a more in - depth way. Another theme is the concept of destiny. How the characters' fates are intertwined with the fate of Middle - earth. And of course, the theme of adventure, as characters embark on new quests within the richly detailed world of Middle - earth.
In these fanfictions, a frequent theme is the breaking of gender norms. Middle - earth has its own set of traditional gender roles, and the crossdressing reader character challenges those. Also, there's often a theme of adventure. The crossdressing doesn't just stop at a change of clothes, but it can lead the character into new adventures, like infiltrating an enemy camp disguised in a different gender's attire, adding an extra layer of intrigue to the story.
You can try fanfiction websites like Archive of Our Own (AO3). There are a large number of Lord of the Rings fanfics involving dragons on it. Just use the search function and type in relevant keywords like 'Lord of the Rings' and 'dragons' to filter out the fics you might like.
One common theme is the idea of an outsider. Jon Snow was an outsider in Westeros, and in Lord of the Rings fanfiction, he could be an outsider in Middle - earth as well. Another theme could be the battle against great evil, just like he fought the White Walkers in his own world, he could be fighting Sauron in Middle - earth.
The theme of adaptation is quite common. The OC has to adapt to the elven society if they end up in Lothlórien, and Haldir has to adapt to having this new, unknown person in his world. There can also be the theme of heroism. They might face a threat together, like a group of orcs invading a peaceful area. Through their combined efforts, they become heroes in their own right, and this can strengthen their bond and also be a central part of the fanfiction's plot.