I think at least allowing allied city states to function as airbases is reasonable. In the real world we see this all the time, city states like Bahrain, Qatar, Djibouti and so on all have vast military bases that play host to foreign powers.
From a gameplay perspective it offers more options. In one game I had a gap between 2 cities I could not move aircraft between, so I had to invade and occupy Hanoi just so I could use it as a transit station. It was an ocean gap and Hanoi was the only island in between, so creating my own city was not an option.
With regards to allies (non-CS), I would like to see some sort of 'lease' agreement. Basically you ask the AI if you can base aircraft at a specific city and offer GPT for the privilege.
I know its off topic, but when will we see Airbase tile improvements like the ones in CTP? Or is there a licensing/royalty issue that prevents Civ from implementing ideas from other games? In Civ IV we could use forts, but that is not the case with forts or citadels in Civ V