There is nothing in the game that says what each vehicle category is, just their general weight and size which prohibits how much armour they can carry and what weapons can fit into them. Whether it's rolling on wheels, chugging on tracks, flying on jets, walking on hydraulic legs, or even hovering thanks to anti-gravity plate, is all up to the player and their imagination and the story they are telling.
The ground combat model only understands the concept of the front line, support area and rear area because it supports both a handful of space marines shooting it out on a tiny asteroid as well as fifty million soul strong combined arms army group pulverizing a Super-Earth planet.
Which means that there is no point in determining whether a specific unit walks or swims or flies. All vehicles can achieve breakthrough already - whether that means German tanks driving through the Ardennes forest or American helicopters flying over Vietnamese jungle is mechanically meaningless. There already is a weapon that can reach enemy Rear Area and multiple weapons that can reach Support Area.
Time-wise, it would be much better for Steve to fix planetary support fighters, AAA and STO, as well as give NPR/spoilers the ability to use them.
Of course, if you can come up with a new, novel mechanic that adds to the ground combat model, then, by all means, do write it all down - but adding planes as a ground unit alone is a waste of time. Not to mention that you would have to make all sorts of exceptions to it to explain how helicopters/aeroplanes can operate on airless rocks in space as well as Venusian planets.
