One common theme is unrequited love. Sometimes in fanfictions, Sif might be pining for Thor while he is oblivious or preoccupied with other things. Another theme could be adventure. They are both warriors, so stories often feature them going on quests together, fighting side by side against various enemies.
You can try fanfiction websites like Archive of Our Own (AO3). It has a vast collection of fanfictions, and you can simply search for 'Thor x Sif' in the search bar. There are likely to be many different stories with various themes and interpretations of their relationship.
One great fanfiction could be about their married life in Asgard. It might focus on how they handle the politics and responsibilities together. For example, they could be facing a threat from an old enemy and their marriage is a source of strength in dealing with it.
The fact that they are both strong characters. Thor is a powerful god, and Sif is a fierce warrior. Their relationship as a married couple can explore different power dynamics. For instance, in some fanfictions, they might train together and their marital bond affects how they spar.
One common theme is the reaction of the Asgardian community. Since Thor and Sif are important figures, their pregnancy could have a big impact on the society. Another theme might be how they handle the challenges of impending parenthood while also dealing with their duties as warriors and protectors.
You can try searching on fanfiction websites like Archive of Our Own (AO3). There are often a wide variety of fanfictions about different characters and scenarios. Just type in 'Thor and Sif pregnant' in the search bar and it should show relevant results.
One common theme is friendship. The OC often becomes friends with the SIF characters and they go through various challenges together. For example, they might help each other prepare for a big concert in the SIF story.
When writing the 'lemon' aspect, if it's appropriate for your story, be tasteful. Focus on the emotional connection between the characters during these scenes. Use descriptive language to show their feelings rather than just the physical actions. Also, the setting is important. You could set it in a secluded part of Asgard, like a forgotten garden or a hidden chamber. This can add an element of secrecy and intimacy to their encounter. And don't forget to have a good plot that leads up to and follows these moments, so the story doesn't feel like it's just about the 'lemon' parts.
Obviously, Naruto and Lady Sif are the main characters. Naruto is a ninja from the Hidden Leaf Village, known for his determination and powerful jutsu. Lady Sif is an Asgardian warrior, strong and skilled in combat.
One common theme is self - discovery. The FEM OC often has to figure out who she is within the SIF world. For example, she might discover new powers or abilities she didn't know she had.