A common trope is the 'forbidden love' aspect. Since Future Trunks has a lot of responsibilities and his situation is often complex, falling in love with a Saiyan can be seen as something that goes against his duties at times. Another trope is the 'power couple' idea. As both are Saiyans or related to Saiyans, they are often depicted as being extremely powerful together, both in combat and in their relationship.
One great fanfiction in this theme could be 'Trunks' New Heart'. It beautifully depicts Future Trunks' journey of falling in love with a Saiyan. The author does an amazing job of exploring his character in the context of new emotions.
One common trope is the 'Outcast Attraction'. Since Remus is a werewolf, he is somewhat of an outcast. Hermione, being the empathetic character she is, is drawn to him because she sees beyond his condition. This creates a sort of 'I'll save you' dynamic.
One common trope is the 'slow - burn' where Ladybug gradually realizes her feelings for Chat Noir over time. Another is the 'identity reveal' leading to a new level of closeness. And the 'damsel in distress' where Chat Noir saves Ladybug and it sparks new emotions in her.
One common trope is the 'enemies to lovers' situation. The girl could be initially at odds with Dean due to a misunderstanding or different goals, but as they interact more, love develops. For example, she might be part of a group that Dean's initially investigating, but then they realize they're on the same side in a bigger picture.
A less common but still present trope is the 'memory lane' trope. The bully and the victim might recall past incidents, but now with a new perspective because of the bully's feelings. For example, the bully might remember a time when they were especially cruel to the victim and feel remorseful, and the victim might remember small acts of kindness from the bully that they didn't notice before. This trope helps to further develop their relationship and the overall story.
Well, often you'll find intense power dynamics and a lot of internal conflict. The tyrant might struggle to balance their feelings with their usual authoritative nature.
One common theme is time-travel. Since Trunks is known for his time - traveling adventures, many fanfictions explore this aspect further, like dealing with the consequences of changing timelines.
One common trope is the 'forbidden love' aspect. Since Haku was initially an enemy, there's that sense of it being a relationship that shouldn't happen. Another is the 'healing love' trope, where Naruto's love helps Haku overcome his past traumas. Also, 'discovery of true self' is often used, as Haku may find a new purpose and identity through Naruto's love.
A frequent trope in such fanfics could be the 'shared trauma' trope. The siblings might have experienced something really horrible in Gravity Falls, like an encounter with a particularly evil creature or a dark mystery that they can't quite shake off. This shared trauma binds them together in a way that's different from normal sibling love, and it might have some dark undertones. For instance, they might be codependent on each other because of this trauma.
One common theme is time - travel adventures. Since Future Trunks is known for his time - traveling, fanfictions often explore different time - travel scenarios involving his female counterpart. For example, she might go back in time to change certain events in the Dragon Ball timeline.