Everyone's living in a dream world.
World geometry uses baked lightmaps. We haven't seen any examples of fully dynamically lit world geometry (terrain and non forge geometry) yet in Halo 3, we're not going to see it now.
There's also no way that they added a full terrain editor into Forge in the last title update.
The reason why Foundry and Sandtrap are in dark lighting conditions is because Forge objects don't cast shadows, and when you have large objects such as structures that don't cast shadows it looks horrible in a fully lit environment.
I don't think you'll be seeing different skybox options because of the lighting problems.