Currently in the sim, especially for aircraft whose cockpits are lighter colors, the exterior environment map affects lighting (and reflections) inside the cockpit, so that the pedestal/floor is illuminated blue during clear weather, and the overhead panel/ceiling is illuminated green/brown from the terrain. This is not realistic, and being able to use a custom interior map to stop exterior lighting from affecting texture illumination is necessary.
Furthermore, lights from airport scenery or ground services (especially the ground power unit/truck for airliners) produce light that "bleeds" into the cockpit and illuminates surfaces which should never receive their light. Being able to define some sort of occluder to completely block this out is also necessary.
Also, being able to create custom cubemaps/reflection maps for the interior of the aircraft (such as the cockpit) would be a huge plus, to make realistic screen reflections.