We need and extra interface/panel for the variable fonts.
All the 'visible' axes need to populate that, so we can fiddle around with these and get in realtime visible output on the pages.
How these interfaces must look like? Maybe, for a start, just as Illustrator has now, with sliders?
Later, more refined controls. Some axes are just on/off switches (checkboxes). Maybe a font with a two axes designspace should have a square view where the user can drag a location in and so control the two axes at once... (thinking out loud here)