This seems like a really neat possibility. I can see the use for having various "flavors" of a theme and having the ability to switch back and forth. Specifically, I'm imagining a multisite network where each site could pick between several designs this way. Seems much more efficient.

The one current hangup I have with theme.json is that I haven't found many boilerplates for it yet. A generator with a GUI would be wonderful to have (apparently the first one, gutenberg-theme.xyz, is no longer online).

But JSON is a different animal, and those of us used to CSS would really benefit for an easy way to add/remove/tweak all of the possible features.