What am I looking for? Ideas, really. I could dig up my old world builder's guidebook and/or go through the steps of the WotC setting search, but mostly looking for a few ideas to develop.
Looking at what I am already thinking... I don't want to set anything in stone until I set up some "world hooks", but to expand on my beginning thoughts.
1) Making room for d20 products.
One thing I notice about my current setting is that I typically have little room to use many d20 system products because my world is so strictly defined as it is. I am looking to create a world that is, perhaps, more open ended, or plays to stereotypes often used by d20 system products.
As mentioned, two things I was already considering facilitating are Bluffside and Necropolis.
Bluffside's background situates it on a northern continent that was more or less abandoned after a asteroid strike. The northern continent here is situated so that migration might have been possible at one time. Simple, but there you go.
As for necropolis, as I already mention, I intend the gap between the first and second continent to be a mediterrainian-like area, where I could situate an egypt like state.
Other items I am looking at integrating:
Some races from Mythic Races and Mercenaries, Deities from various books, etc.
2) Less/better distributed earth analogs. Arguably my current world is divided into regions that arguably mock ancient earth... i.e., I have the analog of an ancient Orient, analogs of europe, etc. What I would like to do is mix up cultural archetypes accross the continents so at the very least it doesn't so strictly resemble an analog of ancient earth, and so I can insert certain cultural elements in a variety of places, and so I have more distinct cultures.
3) A mix of the familiar and the exotic. The whole point of this effort is to make something a little different than what I have been doing, so I want to strive for a few exotic elements. But it still has to be D&D, and I don't want it to be so strange that the players don't have a point of reference.
4) Themes - like I said, I may draw out of the hat for world hooks, but I am already leaning towards a bit of a "technomagic" world hook, at least for one of the continents. This may leave me with an Iron Kingdoms or Final Fantasy feel.
I also have a fondness of vance and was considering making wizards more vancian in feel in this world, and also gave thought to the idea that large parts of the continents could be untamed and barbaric, like in Conan novels. Another author I might like to draw from is Clarke Ashton Smith.
5) Background. What makes my existing campaign such a long-running success is the depth of the backstory, and motivations of existing powers. I have almost no ideas what to do here yet, but looking forward, I would like to link a lot of implications and inter-relations into the background as it is built.
One possible background point that I can build from. I intend to make this world one of "the eight", eight worlds that are nominally connected by secret gates that my other game worlds (well, two of them at least) are part of. Who created this "connection" and why is a mystery, but they have played a role in settlement patterns in the worlds, and may play a role here as well.
One thing I was thinking is that the northern continent is home to "formian" type giants and other horrific creatures. In my campaign world, I have a psuedo-celtic culture that supposedly fled to my campaign continent from such an evil. One thing I may examine is making the lost cousins of this race, and create some strange culture derived from divergant cultures from my other Celtic one.
Another possible link to my current world is that my Asia-like area is full of yuan-ti infested jungles, and such creatures fit right in with my Clark Ashton Smith theme. I don't like replicating ideas unless there is a reason, so perhaps the location of the gate could be in the midst of the yuan-ti jungles. That might even be a means of introducing the PCs... they want to know the source of this yuan-ti empire, and discover an open gate in the middle of their city.
Those are my basic, bare ideas. Perhaps I'll reach into the idea bin saturday and post some random ideas.
Late thought: This yuan-ti thing has me thinking that, at least on one of the continent, humans are not the dominant race.