I'm not what most would consider a Pro user but I've always loved and supported Adobe's application integration (the 'edit with' submenu, smart object, linked item, etc.) So maybe a nice implementation with the eventual integrations with 'send to Muse', or 'convert to Illustrator artboards', etc. might be:

Upon importing an asset or making a symbol, since these may be simply placeholders for actual assets, would be to have the ability to give them a definition of 'type'. This could be a variety of things but if you're prototyping a spot for a carousel but the content isn't yet made or available, upon import this could at least be labeled, or an illustration, or a dynamic web element. Even a gradient, could have options for type-ing as a static image or 'will be' a css render.

A potential advantage is when the export happens, a lot of the routine re-making, optimizing, code replacement might be more seamless.