One possible plot could be that they face unexpected pregnancy. They initially panic but then slowly embrace the idea. Natsume becomes over - protective of Mikan, and they have to deal with the reactions of their friends and family. Maybe they decide to move to a quiet place to start their new family life.
One idea could be exploring how they adapt to the news. Mikan might be initially scared but then excited about the new life. Natsume could be overprotective, making sure Mikan is always comfortable and safe. Another plot could involve their friends' reactions, like how they support or are shocked by the pregnancy.
Their relationship would likely become more intimate and closer. Natsume would be more caring and gentle towards Mikan due to her pregnancy. He would constantly worry about her well - being.
Mikan's character development could involve her learning about motherhood. She could start having dreams about the future with her child and Natsume. This would make her more determined to be a good mother. Natsume, on the other hand, might develop a sense of purpose. Before, he might have been a bit lost, but now with the pregnancy, he has something to strive for. Write about their internal thoughts and how they communicate these changes to each other to show their development.
I recommend 'Mikan and Natsume's Promise'. It's a short but heart - warming fanfiction. It focuses on a promise they make to each other and how they uphold it throughout different situations.
One common theme is love. Writers often explore the romantic relationship between them, from the initial attraction to the deepening of their feelings.
It's Natsume and Mikan. Natsume could be seen as the protector in some fanfictions. He has powers or abilities that he uses to safeguard Mikan. Mikan is the one who gives Natsume a reason to be more than just a loner. The kiss between them is a significant moment that can change the dynamics of their relationship and the story as a whole.
One great mikan sakura and natsume hyuuga fanfiction is 'The Unseen Bond'. It beautifully explores their relationship in a new setting. The author does a wonderful job of portraying their emotions.
I'm not sure specifically about 'lemon' fanfictions (which often imply inappropriate content). However, there are likely many sweet and engaging fanfictions about Mikan and Natsume in general. You could try looking on fanfiction platforms like Archive of Our Own, but focus on the ones that are rated appropriately.