But I have such a good one...
ok, ok. I don't know if you would use it (becuase I have such a good one), but it would be interesting to see a setting built from the ground up based on the core rules. Thoughtfully, carefully, thoroughly, and usefully (to the players and DM) using the implications and implicit assumptions of the core rules.
In other words, a world that works like the core rules imply it actually would work and that provides great support to using the core rules.
One caveat: if this broke some other key fantasy assumption (ie following the core rules castles are not very useful), then it may be ok to put something back into to keep that viable (the really defensible castle spell). But this shouldn't happen that much.
Its worth noting that no setting out there really fits this criteria (including my own). Either they where retrofitted to match the core rules (Oerth, the Realms, Krynn, Kalamar...), or desinged to have a bunch of other stuff going on (Eberon and many, many, others).