I personally second this suggestion.

Would it have to work more like Autodesk Inventor or Solidworks in assemblies where the program tries to find symmetrical parts and will alert you if none are found? There aren't too many asymmetrical lego parts but the ones that do exist are widely used. And would the feature extend to stickers and prints as well?

Man I really wish this was a feature. I've spent about 10 minutes diligently mirroring one side of this complex build I'm working on and a Mirror would be so much easier. I guess it'd be nice if it could function similarly to the Autodesk one but I assume it would be easier to just make it rotate all the pieces 180 degrees and moving them to "mirrored" positions. Either one would be super useful.

I also would like the ability to do mirror operations, it would make symmetrical builds SO much easier.

even if we were limited to x/y/z rather than arbitrary planes, that would be a GODSEND.