If not NPCs, then may I suggest encounters similar to the ENWorld encounter competition? I think the "world cafeteria" idea is a good one...if it were not for the feeling that as a resource, it's redundant.
IMO, there is already way too much setting material on the web and from d20 publishers - I think that this is because people just love designing worlds. It's almost kind of self-indulgent of designers to keep designing worlds (as opposed to other resources) instead of leaving that "sweet spot" to the DM. I think that this is why published settings are so niche - worldbuilding your own little sandbox is fun! (This could even be why it's appealing to you most as an option for basing your web site around - you know it's fun. Hell, ENWorld started this way with Daemonforge, from what I gather...)
Counter-intuitively, if what I say is true, then why do adventures sell more poorly than setting supplements? I think that usually adventures are more poorly written than setting material (yes, I do believe that a good adventure is harder to write than setting details and flavour) and they're often not big enough - or not small enough. In drawing a campaign together from published or online modules, individual encounters/sidetreks or mega-adventures see the most use from me - the former because they're easy to fit, the latter because they're the main event. I suspect that I'm fairly typical in this respect.
Big books of 1-2 page adventures (ala
Wonders of Lankhmar), are something which I've found extremely useful, but surprisingly rare...for whatever reason. Maybe they don't sell?
In short, I think that encounters (or mini-adventures the size of a side trek from
Dungeon magazine or less) would be far more useful to the average campaign than generic world material, because:
a) For whatever reason, they're a lot less common than world material - I think that people love making macro stuff and maps more than they do the nitty gritty - just look at your average campaign home page for proof.
b) Generic world material often ends up as just that - generic. The most interesting world material I see revolves around a certain flavour, and if it's to be slotted into any old sword & sorcery world, often that flavour is lacking.
Just 2 more cents from me. Heh, that makes 4 cents.
