Friendship is a very common theme. The stories often revolve around how Mike and Bella's friendship is strengthened or tested through their experiences related to lemons. For example, in some fanfics, they might have an argument over a lemon tree but then make up in a really sweet way.
Another common theme is discovery. They could discover a new type of lemon or a hidden place related to lemons. This discovery then leads to personal growth for both of them. It's like in 'The Secret Lemon Garden' fanfic where they find a hidden garden full of unique lemons and it changes their perspective on life.
Magic is also a theme that pops up. Lemons in these fanfics sometimes have magical properties. This adds an element of fantasy and adventure to the stories. In 'The Magical Lemon Quest', Mike and Bella use the magical lemons to solve problems and go on exciting adventures together.