If we look at the various settings in terms of how familiar people are with each and how 'fleshed out' they are with different adventures, novels, setting books, et cetera I think the rankings would run something like;
- Toril
- Oerth
- Eberron
- Krynn
- Planescape
- Ravenloft
- Athas
- Everything else
DDO, on the other hand, has a great deal of Eberron content, a fair amount on Toril, the central core of Ravenloft, a handful of disconnected bits of Oerth, and just faint hints of Planescape.
As such, I'd like to see more of the popular materials from Toril and Oerth brought into the game... at least approaching parity with the amount of Eberron material (which would take years). That might pave the way for a bit more Planescape to help connect the various game realms into a more coherent whole. Expanding Ravenloft and/or adding content from Krynn, Athas, or other settings could be individual small projects, but it'd be a long long time before I'd expect to see significant coverage of those.