Well, the following can only be modded, but I would love to see this done via the missionfile.
So that you can create your own face with a custom texture from the missionpath and load this texture multiplayer capable on a unit.
E.g. something like
this setFaceTexture "faces\bruceWillis_co.paa";
But maybe also with a cfgfaces entry in the description.ext, so in the way it is modded. I don't know how this could work best. Anyway, that would be cool for some custom characters in missions, without extra mod dependencies needed.