Shared Control
If someone is gone away on vacation, for example, and won't be able to manage their airline, shared control would be a perfect solution, as players would be able to give control of their airline to another player. Basically I think that the system Airline Empires has is pretty good, but we'd have to tweak it since we only have one airline per player. Here's how Empires' system works, however I know that we'd need some sort of modification:
Each player is given a number (for example, mine is 84154) - I believe it is assigned when you create your account, so older accounts have lower numbers.
When you want to give control to someone else, you put their number into a spot on your airline's page, and then they have full control over your airline, just as much as you do. However, you can kick them out, by removing their number, or typing in another player's number.
I think this feature would be a good addition to the game.