I would like to use SPV3.3 as an example of the sheer level of customization available in the Halo CD engine. Some examples of customization include
Skybox Customizations (I.E. On missions like Halo, Silent Cartographer, Maybe Assault on the Control Room)
BDU Customization (I.E. Our player model in Halo CE. Being able to switch around between playing as the Master Chief and an ODST would be incredibly awesome)
Cortana Customization (Being able to change her character model would also be awesome).
not sure if it is the case, but it is possible that the one map Gearbox didn’t quite get ready for release is that one since it went by the name “indoors”