Felk Mor.
A superdungeon campaign up to level 10, including a vast subterranean cavern miles wide that houses different clans of humanoids, each with a decent sized town. Depending on which clan the PCs ally with (if any), they can use one of the towns to act as a base of operations for the temple dungeon proper rather than need to make their way back up to the surface each time.
Think of Journey to the Center of the Earth meets Cthulhu, as the main plot line is centering around demonic cults. I've posted some of the monsters here (mushropod, mrav covjeka, etc).
However, just because it's a subterranean superdungeon doesn't mean there are drow, mind flayers, or beholders. There are none. On purpose
House Rules: Healing. On a full rest, you get back 0 hp, but you do get back all of your HD.
Spoilers!!!
Primary tangents (only primary ones; there are dozens of minor ones since this is a pretty sandboxy campaign):
* ally with the lizardfolk and go on an adventure to explore their ruined city they fled from
* ally with the orc tribe and help them reclaim their surface home back from the gnoll raiders
* follow the rumors to the mountains and encounter the young red dragon and his bugbear followerers. Possibly ally with bugbears to overthrow the shaman and help against the dragon.
* rescue Markis and help bring him to his rightful place as king (this one could turn into a huge campaign on its own)