There's 'Clash of Perspectives' which is a great 'harry and louis fight fanfic'. In this one, their fight stems from different career goals and how they each think the other should be handling their public image. It's a well - written exploration of their characters in the face of conflict.
One idea could be an Acromantula befriending a Hogwarts student who is an outcast. The student discovers the Acromantula has a unique ability that could help in the fight against Voldemort. So they team up secretly.
One sad one - sided love story could be like this. There was a guy, Mark, who worked with a girl, Anna. Mark was head over heels for Anna. He would bring her coffee every morning, offer to do extra work for her. But Anna only saw him as a good friend. She talked to him about her dates with other men. Mark just had to listen and smile, his heart aching inside. In the end, Anna got engaged to someone else, and Mark was left with a broken heart.
Steve McGarrett is a great character to focus on. He's the leader of the team, with a strong sense of justice and a lot of backstory to explore, like his military past. Another good one is Danny Williams. His banter with Steve adds humor to the show, and his family - man side gives room for some emotional depth in a fanfic. Kono Kalakaua is also interesting. She's a strong female character with her own set of skills and a connection to Hawaiian culture that can be delved into.
Adventure is another theme. Since they have both been in movies with adventure elements, like 'The Lord of the Rings' series, fan fictions might create new adventure stories for their characters, perhaps sending them on a quest in a different world or time. For example, they could be on a journey to save a magical realm that is in danger. This allows fans to expand on the adventurous side of their characters.
One common theme is the clash of different power systems. The magic in Harry Potter and the supernatural powers in Underworld are very different, so seeing how they interact is a big theme. For example, how would a wizard's spell work against a vampire's strength? Another theme could be the exploration of different cultures within the two universes, like the wizarding culture and the vampire/werewolf culture.
It might imply delving into a complex or mysterious situation. Say in a corporate scandal. 'Watch the deep' could be about closely examining the core of the matter, and 'the inside story' is all the behind - the - scenes details, like who knew what, when they knew it, and how they tried to cover it up.
Another is within the family. Some gays have to endure the horror of being disowned by their families when they come out. Their own parents might cut off all communication, leaving them feeling completely alone and abandoned in the world, which is a truly heart - breaking situation.