I'd love to see it but I don't have faith in ANY developer to make it balanced. There are just too many things in an engagement between a flier and ground troops that simply cannot be balanced around. I don't think it's even achievable in truth.
That said, if someone DID achieve it, I'd love for it to be included.

Yeah, flying is a completely different play style that needs its own goals and strategies. It's also probably a nightmare to keep balanced, with infantry, vehicles, and aircraft one of them is going to end up dominating the other or one is going to feel really useless.
I remember in Planetside 2, tanks would usually be quickly thwarted by anyone bum-rushing them with explosives.

Introducing air power will do the same thing to this game that it did to the real world: turn the entire war economy into a race to have the most planes.

ehhhhhhh no. IRL planes bomb from ridiculous heights, heights that frankly are not possible in this type of game. Take PS2 for example. Pull a few skyguards and say bye bye to an air zerg. Well, that or have squads with lockons flank the current air zerg's location.

Yes, add this please!

Do a little reading into the history of aviation. For decades after their introduction in WWI as military vehicles, airplanes were extremely dominant - all the while flying at relatively low heights and speeds.

And I'm telling you that RL =/= video games. PS2 is a decent example of this.

it didnt really work out in planetisde 2 which is a good way to judge how it would work in a game like eternal crusade which is similar.
You either build and balance the game from the ground up with air combat as a large part of a game or it will be either annoying for ground troops or frustrating for pilots (who purchase an expensive aircraft only to see it shot down by some anti air spam).

Aircraft have been a balance problem in Planetside 2 since beta. The history of buffs, nerfs, and hard counters is a sordid affair that the developers of EC should take as a cautionary tale.

Well yeah, but that shouldn't stop them from implementing flying vehicles. I would say that in their current state, they're better off than they have been in months past.