There may be occasions when the user may want to modify the design-space coordinates of a given variable font globally on the document. Do you think this is a valid concern / valid usage scenario ?

In that case, a variable font instance may behave like a shared resource, analogous to a shared gradient or filter-effect. But then we would need to make this an explicit user-choice, perhaps by providing a dedicated dialog for setting up such shared font resources. Like a "palette of font instances". Does it make sense ?