Re more detailed metadata, I think it's way to early to be guessing at what the best future UI affordances for axes may be.
As John indicates, there is always some assumption to be made when no flags are set. If we don't explicitly say what should be assumed, then different apps will do different things. Also, note that fonts are starting to get created that don't have any flags set, and so the no-flags interpretation should keep those fonts in mind. Do we treat that as the "never show" case, or the "only advanced" (by some determination) case? I think the safest thing is to treat that as "always ok to show" on the grounds that users are less likely to be confused by a variable font that has multiple knobs/sliders than by buying a variable font advertised as supporting axes but then they can never see them in apps.