Spoiler: Here are some MODELDEF examples from Embers Of Armageddon...Here, you can see, per actor, a model (with subsequent skins) is defined for use, and is assigned to a specific sprite frame (A or B) and index (TRAC). When that specific actor shows up, then while it's in a state that displays the listed sprite index + frame, the model will show up in place of the actual sprite.
My proposal is to do something similar with sprites themselves. We map graphics to sprites in a similar way. You take a graphic, and any actor that wants to have that graphic show up in place of their own listed sprites will have it mapped to that sprite frame (even down to specifying rotation frames). This way, when a specific actor displays a specific sprite index and frame, it will show up as the assigned "skin". You can use the same sprite index and frame across different actors which have different "skins" associated with them to display different graphics, much like how those tracer actors show up differently despite using the same index name and frame letter.
This would help with incorporating a "skin system" for sprites, which also allows for simpler actor definitions by allowing the use of a proxy sprite index name + frame letter instead of directly referencing a sprite graphic. This could also help with resolving sprite conflicts, as this could (in theory) allow for an infinite number of sprites to be present in the project with no overlap.
What are your thoughts?