I'm not sure I understand the whole "RPG/fighting/fantasy doesn't fit South Park" (yeah, I'm simplifying and conflating together different gripes from different people, so yeah, I'm probably missing some nuances from some posts here, but this is an issue that crops up in other places, not only on the Obsidz forums) when actually those are themes that have been touched in plenty of episodes. There have been entire episodes dedicated to the boys suddenly ending up in the realm of imagination and fighting alongside Gandalf, Jesus, etc. , and I don't see any reason to believe this game will be different in that respect. I mean, I obviously don't know whether it will work mechanic-wise and how well the game will come out, that depends on Obsidian, but I'm fairly sure it'll be true to the source material considering how involved the license holders are (they basically approached Feargus and his minions themselves and are working hand in hand with them).
That said, while I can see something good coming out of this, I can't say I'm personally excited, because I appreciate Obsidian mostly for their slightly less pulpy and more intricate writing and their attention to quest design/C&C/etc. (which doesn't always provide good results, mind you, but always pique my interest), and this doesn't really sound like a project where they'll push that. Hope I'll be wrong though.