Well, you might also try some fanfiction communities dedicated to Adventure Time. They could have some great stories about Finn being heartbroken. Sometimes, on Tumblr as well, fans share their own fanfictions or reblog interesting ones. You just need to search for the right tags like 'Adventure Time Finn Heartbroken Fanfiction'.
Betrayal is also a theme that can be found in this type of fanfiction. For example, if a character Finn trusts deeply turns out to have been lying to him or using him, it would break his heart. And then there's the theme of growing up and the pain that comes with it. Finn realizing that the world isn't as simple as he once thought and losing some of his innocence can be a source of heartbreak in these stories. These themes help to explore different aspects of Finn's character and his relationships in the Adventure Time universe.
One popular 'Adventure Time Finn Love' fanfiction is 'Finn's True Heart'. It delves into Finn's journey of understanding love in a really cute way. Another great one could be 'Finn's Love Quest' which has him going on various adventures to find his true love.
One place to start looking could be Archive of Our Own (AO3). There are often many creative fanfictions there. Just search for 'Adventure Time female Finn' and you'll likely find some interesting reads.
There's also a fanfiction where Baby Finn discovers a magical artifact. In this story, he doesn't fully understand what it is but his curiosity leads him on a journey. The author does a great job of portraying Baby Finn's naivete and how the other characters react to his child - like behavior. It's really cute and full of the charm that Adventure Time is known for.
One great piece of Adventure Time Dragon Finn fanfiction is 'The Dragon's Bond'. It delves deep into Finn's relationship with his dragon side, exploring how he balances his human nature with the newfound dragon powers. Another is 'Finn's Dragon Adventure', which takes Finn on an epic journey where his dragon form is crucial in facing various challenges in the Land of Ooo. And 'Dragon Finn and the Magic Crystal' shows Finn as a dragon protecting a powerful crystal from the evil forces in the land.
Sure. 'Finn and Marceline: The Nightmare Realm' is an interesting one. In this fanfic, they get trapped in the Nightmare Realm and have to fight their inner demons while also relying on their bond to escape. And then there's 'The Crimson and the Hero' where Finn and Marceline team up to face a new threat to Ooo that is related to Marceline's vampire past. It shows how their different personalities work together in a time of crisis.
One great 'Adventure Time Finn the Demon' fanfiction is 'Finn's Demonic Adventure'. It explores Finn's encounter with a new kind of demon and how he overcomes the challenges. It has some really interesting plot twists and character development.
There is a really interesting piece of fanfiction where Finn has a romantic adventure with a new character named Lily. This fanfiction is unique as it not only focuses on the romance but also on how Finn's relationships with his friends in Ooo are affected by this new relationship. It's a long and detailed story that really shows a different side of Finn's character in terms of love and friendship.
One popular Finn Adventure Time harem fanfiction is 'Finn's Multiple Journeys'. It explores Finn having different adventures with multiple characters in a harem - like scenario. Another is 'Finn and His Friends in a New World' which also has elements of a harem concept with Finn interacting in unique ways with various characters from the Adventure Time universe.
I really liked 'Ice Finn's Frozen Quest'. It has Ice Finn going on a journey to find a magical ice crystal that can restore balance to the Ice Kingdom. It's full of action and some really cool descriptions of the icy landscapes.