One common theme could be the challenges she faces during pregnancy. This might include dealing with her own emotions, the reactions of those around her, like her friends and family. Another theme could be the relationship with the father of the baby, if it's part of the story, exploring how they support or sometimes struggle to support each other.
Another theme might be the anticipation and excitement of a new life. It could explore Mary Jane's dreams and hopes for her unborn child. Also, there could be themes related to how her pregnancy affects her daily life, her career if she has one in the fictional world, and her friendships. Maybe she has to rely more on her friends during this time, and the fan fiction could show how those relationships evolve.
Well, the theme of growth is quite common. Both Peter and Mary Jane grow as individuals and as a couple in many fanfictions. The theme of support is also there. Mary Jane supports Peter in his superhero endeavors, and Peter supports Mary Jane in her acting career. And of course, the theme of danger, as Peter's life as Spider - Man often puts Mary Jane in danger.
One common theme is the struggle between the human and vampire worlds. Mary Jane often has to navigate the differences between her normal human life and the strange new vampire - influenced world she encounters. For example, in many stories she has to figure out how to deal with vampire powers and politics.
Love is a very common theme. Since Peter and Mary Jane have a long - standing relationship in the Spider - Man lore, fanfictions often explore different aspects of their love, like how they first fell for each other or how they keep their relationship strong despite all the challenges.
Love and sacrifice are common themes. Since Peter has his responsibilities as Spider - Man, he often has to sacrifice time with Mary Jane for the greater good. And Mary Jane has to love him despite his absences and the danger that comes with his superhero life.
In some Spider - Man fanfictions, Mary Jane being pregnant can add a whole new layer of drama. It could mean that Spider - Man has to balance his superhero duties with being a soon - to - be father. Maybe he'll worry about protecting his family more than ever, and there could be new villains who target his unborn child or Mary Jane to get to him.
Since 'lemon' often implies more mature or sexual themes, this is inappropriate content and I can't discuss common themes related to it. However, in regular Peter and Mary Jane fanfiction, common themes are their relationship, Peter's double life as Spider - Man, and how Mary Jane deals with it.
The theme of sacrifice is also prevalent. Peter may have to sacrifice his time with Mary Jane to save the city. In many fanfictions, Mary Jane has to sacrifice her normal life, like not being able to have a completely ordinary relationship because of Peter's double life. Also, the theme of growth is common. Both Peter and Mary Jane grow as individuals and as a couple throughout the fanfictions.
A major theme is the exploration of their individual strengths. Mary Jane has her own set of skills such as her acting ability and her ability to read people. Tony Stark has his engineering and scientific genius. Fanfictions often bring these together. For example, they might work on a project where MJ's acting skills are used to deceive villains while Stark creates the tech for the operation. Also, the theme of personal growth is common. They both grow as characters through their interactions with each other in the fanfictions.
A common theme is jealousy. Since they both have a connection to Spider - Man, there's often jealousy between Black Cat and Mary Jane. Another theme is the contrast between their lifestyles. Black Cat's life of crime and adventure compared to Mary Jane's more normal life.