I think I like this @Boviz
Reading your post the first time, I thought "Yeah, that makes sense." But then I hit a wall: implementation.
Because symbols live in the UI Library (and can be added to your project via Quick Search), wouldn't the alternates live there too? If so, wouldn't it be easier to just duplicate the symbol in the symbol library, make your changes, and then save it as a new symbol?
If the symbol alternate lived within symbol itself (which I don't think is something we could do - but let's pretend we could) would that really be more convenient? You would still have to edit the symbol, add it to your canvas, and then select the alternate version you wanted to do. I'm not sure that's an easier workflow than just duplicating and editing.
So I'm in this weird spot. My gut reaction to this was "yes, that would be cool" so I feel like I am just missing a use case. How do you envision this being implemented? Am I missing the obvious?
Let me know!