I can't really say for sure as I don't actively read lemon fanfictions. But generally, in any fanfiction about two characters, themes like their relationship development might be there. Maybe how they support each other in different situations in a more romantic or intimate context in the case of lemon fanfictions. However, this is all speculation.
It's difficult to precisely define the common themes in such fanfictions without actually delving deep into them. However, it could be that the authors might explore the idea of how Natsu and Wendy's powers interact in a more emotional and perhaps physical context in lemon fanfictions. But this is a very unorthodox way of looking at the characters, and most fans would prefer to focus on their positive and healthy relationships as shown in the original series.
A prevalent theme in 'natsu and wendy married fanfiction' is the exploration of their emotional connection on a deeper level. Since they are now married, the stories might focus on how their love has evolved from friendship or camaraderie. There could also be themes related to cultural or magical elements specific to their fictional universe and how these play into their married life, such as using their magic in unique ways to create a home or protect their relationship.
Friendship is a very common theme. Writers often explore the bond between Wendy and Lemon, how they become friends and how they maintain that friendship.
One common theme might be a 'what - if' scenario where Natsu and Wendy have a closer relationship than in the original series. But since 'lemon' implies mature content, it's not really in line with the general Fairy Tail spirit. The original series is more about magic, friendship, and guild loyalty.
I'm not entirely sure about the common themes in 'wendy and mest lemon fanfiction' specifically, but in general, love and relationship development might be common.
One key theme could be friendship. Natsu and Wendy have a special bond in the original series, and in fanfics, it might be explored further. For example, they could go on adventures together and strengthen their friendship through facing various challenges. Another theme could be growth. Wendy is still young and growing, and with Natsu's more experienced presence, fanfics might show her learning and becoming stronger. Also, themes of loyalty often come up. They are part of the same guild, and their loyalty to each other and the guild can be a central part of the story.
Power and growth is another theme. As they face challenges together, they become stronger both magically and as individuals. Their relationship also strengthens during these times. They rely on each other's powers and learn to work in harmony, which is often explored in fanfictions.
One common theme is friendship. Natsu and Happy's bond is often explored further in these fanfictions through their shared experiences with the lemon elements. Another theme is adventure. The lemon might be a catalyst for an exciting journey or quest.
One common theme could be the exploration of their romantic relationship in a more intimate way, which is what the 'lemon' aspect often implies. Another might be the growth of their bond against various challenges.
One common theme could be adventure. Since Natsu is often associated with action - packed scenarios, it might be carried over into the fanfiction. Another could be friendship, as relationships between characters are often explored in fanfiction.
One common theme could be the exploration of their relationship in a more romantic and sometimes passionate way. Since it's 'lemon' fanfiction, there might be themes related to physical attraction and intimacy.