ZDoom

I was thinking that the child list would start off where the parent list ended so you could still specify more paths and frames or whatever it may be

From what I understand, this is not possible due to the way MODELDEF is parsed internally. The advantage of the current system is that you can change path, models and (surface)skins at any point in the MODELDEF lump, for example in between two FrameIndex lines. This would not be possible with inheritance.

Would it be easier to inherit from a parent to change a skin than to copy 100 frames to do so?