Another theme is redemption. Ice King, being a somewhat villainous character at times, often goes through a journey of redeeming himself in relation to Finn. This can involve him changing his ways and becoming a better friend.
One interesting fanfiction could be where Finn and Ice King go on a journey to find a magical item that can thaw the Ice King's frozen heart in a more emotional way. Finn's sense of adventure and the Ice King's zany personality would make for a really fun and unique story.
A great 'ice king x finn fanfic' is 'Ice King's Redemption'. This story is about Ice King trying to make up for his past misdeeds towards Finn. It shows his growth as a character and how Finn gradually comes to understand and accept the Ice King in a new light.
Another option is to check out some Adventure Time fan communities on social media platforms like Reddit. There are often threads where fans share and discuss their favorite fanfictions, including those centered around Finn and Ice King. You can ask for recommendations in those communities as well.
The unique element is often the exploration of Finn's different personality in his ice form. He can be more cold - hearted in a literal sense, which creates interesting conflicts.
The appeal lies in the unique combination of their characters. Finn's adventurous spirit and Marshall's laid - back vampire charm create an interesting dynamic. It's fun to read about how they interact in different scenarios.
One popular 'Adventure Time Ice Finn Fanfic' is 'Frosty Adventures of Ice Finn'. It takes Finn in his ice form on a journey through the Ice Kingdom, facing new challenges and making strange allies.
One interesting Ice Finn fanfiction could be about his adventures in a frozen realm that wasn't shown in the original series. Maybe he discovers a new type of ice creature and has to form an alliance with it to save the day.
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.