There are so many players that start airlines and then they eventually abound the game or go financially on the red. Would be great to have the possibility to buy those rather then loose those routes or aircrafts. In this way you could expand your airline and make it a big group.
This could be achieved or either people putting their airline for sale, or after a certain period of inactivity from the user the airline could be acquired by another airline. All rules depending on the world you're playing would still be applicable, you could run both airlines individually but as a big group you could invest money towards each other.
Just a though...