So, if they're not going to provide us with means to greatly alter the underlying assumptions about magic, technology level, etc. (and I maintain that they probably shouldn't do that; they're not making d20 Fantasy, they're making D&D), what can the game really do to support worldbuilding? Frankly, I'm not sure. If anyone has ideas, I'd love to hear them.
D&D is a subgenre of fantasy.
Worldbuilding advice: How to build a world such that it is good for roleplaying (as opposed to LotR). What's the point of including ancient civilisation that were rich in magic but have now fallen? What are good ways of placing monsters so that low-level and high-level chars can adventure in the same setting? What sort of political relationships between nations are best for different kinds of games (political intrigue, dungeocnrawling)? Is there any point to adventure guilds and such? How can I make it easier to get PCs together when starting an adventure?
There are topics to be handled. I can come up with more and answer some of those.
In other words, show us where worldbuilding and mechanics might intersect, and when they do, show us the math behind those mechanics.