Finn might initially go into denial. He could keep expecting Jake to just pop back up like he always does. Then he'd probably get angry, at himself or at whatever caused Jake's death. Eventually, he'd start to accept it and try to honor Jake's memory by being a better hero.
One interesting Finn and Jake fanfiction could be about their adventures in a new, uncharted land within the Land of Ooo. Maybe they discover a tribe of magical creatures that they have to befriend to get a special item for Princess Bubblegum.
Adventure is a very common theme. Finn and Jake are known for their adventures in the original show, and fan fictions often continue this trend, sending them on new quests.
One great theme is their friendship adventures. Stories that focus on their new quests and how they support each other through different challenges are always interesting. Another theme could be how they grow together as characters over time.
Well, it could be about the kids going on their own crazy adventures. Maybe they inherit Finn's bravery and Jake's shape - shifting abilities and use them to save the Land of Ooo from a new threat.
Finn might become more brooding and serious. His usual carefree and adventurous spirit could be dampened by the dark nature of being a vampire. He might be more withdrawn as he tries to hide his new identity. Jake, on the other hand, could become more mischievous. The new powers might make him a bit more reckless and he might use his vampiric abilities for pranks in a not - so - nice way at times. But deep down, he would still care about Finn and their friends.
In this fanfiction scenario, Finn and Jake as vampires might have enhanced physical abilities. Finn could be even more skilled in combat with his vampire speed and strength. Jake, with his shape - shifting ability combined with vampiric powers, could turn into some really unique and powerful forms. They might also have to deal with the typical vampire problems like avoiding sunlight and craving blood, which could lead to some interesting moral dilemmas as they try to stay true to their heroic selves.
Searching on Google can also be useful. Use specific keywords like 'best Finn and Jake fanfiction' or 'highly rated Finn and Jake fanfiction'. But be careful to avoid any sites that might have malicious content or violate copyright laws.
It could be an old enemy of Finn and Jake who has come back to take revenge on their kids. Maybe the Ice King's descendants or something related to him.
Another interesting one is 'Jake's Family Ties'. It delves deeper into Jake's family, exploring his relationships with his extended family members. It's full of humor, just like the show, and also has some really cool magical elements related to Jake's shape - shifting abilities.