A theme that is quite popular is the exploration of their relationship's development over time. It could start from their early days as friends and comrades in Asgard and then gradually develop into something more romantic. There are also fanfictions that deal with the challenges they face in their relationship, like differences in their personalities or duties that they have to balance. For example, Thor's responsibilities as a prince and Sif's as a warrior might sometimes cause friction in their relationship, which makes for an interesting plot point in many fanfictions.
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.
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.
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.
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.
One common theme is the meeting of two very different cultures. Thor from Asgard and Thranduil from Mirkwood have distinct ways of life. Fanfictions often explore how they navigate these differences when they interact. For example, how they deal with each other's concepts of honor and leadership.
Love and redemption are also common themes. The OC might be a character with a troubled past, and Thor helps them find redemption. At the same time, they fall in love. It's a classic trope that gives an emotional depth to the relationship between the two characters. This can be seen in many 'Thor x OC fanfiction' where the OC has to overcome some personal demons with Thor's support.
Teamwork is also a prevalent theme. Given their individual strengths, in fanfictions, they often have to work together to overcome obstacles. Thor's physical strength and Peter's quick thinking make for a great combination when they team up.
Another theme is friendship or more. Sometimes they start as rivals but end up as friends or even something more romantic. They learn to rely on each other's strengths, like Thor's physical power and Tony's intelligence.
Friendship and camaraderie is also a big theme. Despite their differences, they come together to fight for a common good. In many fanfictions, they start off not liking each other much but as they face challenges side by side, they develop a deep bond. It's like a story of two opposites learning to get along and respect each other.
The theme of power and responsibility also pops up. With Thor being the more responsible and power - holding one, and Loki being the more mischievous and power - hungry at times, fanfictions play around with how they balance these aspects in their relationship. For instance, there are stories where Loki has to learn to handle power from Thor in order to protect Asgard.
The theme of mutual respect is also prevalent. Bruce respects Thor's power and Thor respects Bruce's intelligence. In many fanfictions, this mutual respect is what allows them to work together effectively. For instance, in 'Respect and Alliance' fanfiction, their respect for each other is the key to saving the universe from a great threat.