The upcoming South Park RPG was announced only two days ago, but Game Informer readers with early copies of the January issue have already revealed every known detail so far. Obsidian will be developing the game using the Dungeon Siege 3 engine, and creators Trey Parker and Matt Stone have given them 15 years worth of assets from the show to work with. The two creators' influence will really be felt on every level of the game; for example, Parker hates unskippable cut scenes and prefers silent protagonists in RPGs. There are five classes: wizard, paladin, rogue, adventurer, and a fifth that will apparently be made up in-game by Cartman.
The writing will in part focus on making fun of other RPGs and how "big and bombastic" they can sometimes get. It begins when you (the new kid) join a town-wide live action role-playing game (LARP) that gets out of hand (surprise) and becomes a real adventure. The combat is compared to that of the Paper Mario and Mario and Luigi games, i.e. turn-based, non-random encounters with an emphasis on timed button presses. There's also a Final Fantasy VII-esque materia system for augmenting weapons.
Again, Parker and Stone will heavily influence the game—the article mentions them nixing entire missions and game mechanics, like jumping, that didn't seem to fit. Overall it looks like they're taking this project very seriously, both as an RPG and as a piece of South Park canon. Hit the link for a few more details, and expect the game some time next year on PC, Xbox 360 and PS3.