Gloss-mapping
Gloss maps tint environment maps in Morrowind. The original game has them disabled for whatever reason and Morrowind Code Patch has an option to re-enable them.
They do have certain use. While bump maps allow doing a cheap imitation of normal mapping, gloss maps allow doing a cheap imitation of specular mapping in NetImmerse and allow more flexibility in environment map use.
I believe OpenMW could reasonably support gloss maps out of the box for Morrowind meshes: vanilla assets don't use them and newly made assets that could use gloss mapping would really only target Morrowind with MCP applied so there's little possibility gloss mapping use would be unintentional (it's difficult to accidentally assign a black gloss texture to an object and break environment mapping).
Later Bethesda games also commonly implement a form of gloss mapping and call it environment map masking. However, it should not be confused with Morrowind gloss mapping because non-Morrowind models don't typically use spherical environment mapping.
Gloss mapping support would allow modellers to assign gloss maps to environment-mapped objects and expect them to look right in both Morrowind with MCP installed and OpenMW.