Able to change staging outside the designer (I.e on the launch pad)

Seriously. I had enough times of finally testing a new rocket just to blow up at angels 3 because the second and third interstages were on the same stage group.

Not a full on every single part of the current craft on the pad though cause that will be a heck of coding to do. Maybe just the groupings (I.e these mage engines, those side interstages, etc) could be edited on the fly.