One common theme could be Finn's struggle with his identity. In a depressed state, he might question his role as a hero. Another theme could be his feelings of isolation, even though he has friends. For example, he may feel that no one truly understands the depth of his inner turmoil.
Some common themes could be Finn's growth in terms of his relationships. For example, his friendships might deepen or change as he matures. Another theme could be his self - discovery, as he figures out who he is becoming as he gets older. There might also be themes related to his role in the world he lives in, like how his responsibilities increase as he matures.
Well, Finn himself is surely a key character. He's the protagonist and the story would likely revolve around his actions, decisions, and growth. Jake, his best friend, would also be crucial. Jake's unique abilities and their friendship are integral parts of Finn's journey.
Another ending could involve Finn finally finding true love and settling down. He could choose to give up his adventuring ways and focus on building a family, bringing a new era of peace and stability to his part of Ooo. This would be a more heart - warming and domestic end to his story.
Obviously Finn himself is a central character. Jake, his best friend, is also very likely to be included. Princess Bubblegum often shows up as she has a complex relationship with Finn. Marceline might also be in there, as she is an important part of Finn's world and can play a role in his growth and maturity.
Internal monologues are key. The fanfiction could explore his dark thoughts, like his feelings of worthlessness. He might constantly compare himself to others in the Land of Ooo and feel like he doesn't measure up. The author could also show his physical manifestations of depression, such as lack of appetite or always looking tired.
Friendship themes are also common and heartwarming. Finn is a very friendly character. The fanfiction could focus on how Finn and the reader become friends. Maybe they meet at a strange place in Ooo and start to bond over shared interests like music or a love for a particular type of food.
You can try looking on Archive of Our Own (AO3). They have a large collection of Finn the Human x Reader fanfictions that are not of the lemon variety. Just use their search filters to narrow down your results.
One possible reason for Sasuke being depicted as depressed in fanfiction could be his traumatic past. He witnessed the massacre of his clan at a young age, which surely left deep emotional scars. In fanfiction, authors might explore how these past events continuously haunt him, leading to his depressive state. They could show his internal struggle between revenge and finding a new purpose in life.
The Fire Kingdom's Flame Princess could be an ally too. Her mastery over fire could complement Finn's powers in combat situations. Also, the various magical creatures in Ooo, like the gnomes or the fairies, could band together and support Finn. They might have their own ancient knowledge or magic that could be useful to Finn in his adventures with his godlike powers. Finn's kindness towards them in the past might make them more inclined to help him out.