I’ll admit, I’ve done some brainstorming on how Adventure Time could work, either as a board game or tabletop rpg. Nothing serious, just some flights of fancy that never go very far.
The difficulty that Adventure Time, or ANY truly fantastical setting where anything is possible, poses is that rules exist to limit the infinite possibilities of fantasy. Most systems would ground a setting where nothing is grounded.
Let’s take a look at some characters-
Finn is essentially a powerless (or rather “unpowered”) character* whose problem solving abilities, luck, attitude and status as protagonist renders him invincible in what is actually an incredibly deadly and dangerous fantasy setting full of magical monsters and near unstoppable horrors.
Jake, on the other hand, has almost limitless powers as a magic dog with near infinite shape shifting and virtual invulnerability but, unless he has Finn helping him, is pretty easily defeated due to his many character flaws (greed, egocentrism, laziness).
Ice King is a deranged, megalomaniacal wizard hellbent on kidnapping women. In Adventure Time, he’s a creepy joke, but in a more serious gaming setting, he’d be a pretty threatening high level wizard, armed to the teeth with cold and paralyzing magic.
Assorted monsters in Ooo are often either deceptively cute squamous horrors or squamous horrors thru-and-thru. One of the things they often have in common are some very specific weakness or blind spot that must be taken advantage of. Also present are every sort of evil undead imaginable.
All magic in Adventure Time follows rules, but in a self contained manner (i.e. each spell or artifact is subject to the rules that govern that specific spell/artifact, but variety of spells and accompanying rules is pretty boundless). If you can magic it, anything goes, unless someone can outmagic you, either through power or cleverness (like the strategic use of Cantrips in Wizard Battle). Also, magic powers are either inborn or artifact given, though people who’ve acquired them through either means can be wizards; there does not seem to be any distinction.
A diceless system that relied entirely on problem solving would be the best way to accommodate a setting like Ooo. Riddles, puzzle monsters, and negotiations for players to wind their way through would replace typical rolling for searches/traps/actions and punch-face tit-for-tat battling. Capture & imprisonment other than death, go back to start, do not collect 200 gold.
But is this really a “system”? How does one convey the danger, horror and hostility of Ooo, which rivals Carcosa? How does one deal with the threat of PC death in a tabletop RPG that is more Neverhood than Neverwinter? Can play be balanced? Should it? It’s the sort of thing that would only appeal to certain kinds of players, but could be a fun diversion from your typical dungeon crawl. Players and DMs would have to be ready for any possibilities and contingencies within the story and exploration.
*:I’m reminded of reading an article somewhere that went to the effect of “If a group of 1st-5th level characters somehow defeat a Lich, it doesn’t mean they outsmarted the Lich, it means the DM played the Lich wrong.”