It would have to be something that a lot of people here had read, and would be willing to read again or at least join in the conversation. I'm not sure where such a project would stand on the position of spoilers, since it's a re-read, but there's always the possibility some people might be reading it for the first time, too.

Sanderson's work is potentially a good choice, given his connection to Wheel of Time and the fact that we still have a Wheel of Time board.

I'm only starting to read it myself, but I get the feeling that The Dresden Files is something quite a few people here have read.

A Song of Ice and Fire has seen an upswing in popularity due to the HBO series, and was already pretty widely-read here to begin with, but it's probably not quite as universally loved as Harry Potter. Also, it's not finished. But then, neither are the first two suggestions either I suppose.

The novels of Neil Gaiman are quite distinct and easy to read, and there's no fear of spoilers from one book to another. Those works would include Neverwhere, American Gods, Stardust, Anansi Boys, and The Graveyard Book. You could add Good Omens and potentially Coraline as well, and even his two short-story collections, Smoke and Mirrors, and Fragile Things.

Next, Stephen King's Dark Tower series is complete and relatively popular, but the last three books were kind of weird and not that great.

Finally, Robin Hobb has her connected trilogies, the Assassins trilogy, the Liveship Traders trilogy, the Fool trilogy, and her recent Dragon trilogy. I have not read all of them, and I don't know how many people here have.

I'm trying to think if there are any science fiction series that would work, to change things up a little, but all I can think of is either Dune (I haven't read it) or the four related Hyperion/Endymion books.