Just a quick thought that would be pretty exciting to have a ducking and air-born combos?
Like starting combo in air - beat opponent few times, pound him on the ground - few combos while squatting then continue in standing position. More possibilities, strategies and game styles will be unleashed.
Especially with duck hits and kicks, seriously - who using them anyway in KI while they mostly are just single hits?
Cinder already kind of mimics air-born combos. Honestly, KI is a huge mind game on it’s own and having all these different possibilities may over complicate the system and may not be worth it.
Airborne combos can be considered for all characters, since most if not all have a Recapture or can cash out a juggle with a Shadow Ender.
If you specifically mean air-to-air combos, the way the archetypes work in KI means that this mechanic becomes a speciality that very few can use. Cinder and Sadira can easily combo in the air, so can TJ and Orchid with their target combos.
Sabrewulf’s target combo also kinda works as a ducking combo if you start with a d.LP. Aganos’ walking stomp is another example kinda. There are no true ducking combos as far as I know and as with air-to-air combos, it becomes a speciality mechanic.
Maya’s dagger juggles can go from ground to air to ground again and she already has 2 crouching moves that are combo-able, (crouching hard punch with daggers, crouching medium punch with daggers) so I’d say that this was pretty much already a thing? 
I’m getting a weird Eddy Gordo mental image when it comes to crouching combos and I’m not sure I’d want that for every character.