I see there's been discussion before of Airline Mergers and the responses clearly state what the obstacles are to the suggestions provided so far.
I have an idea that might be easier to implement in the game, but I'm a newbie so may be way off base, however I'd like to share my thoughts.
If Airline A and Airline B want to merge, the two payers would negotiate terms. Once agreed to (and perhaps upon Admin approval), Airline B's assets, employees & routes would become part of Airline A. Airline B ceases to exist and a deadline is given to Airline A to divest itself of any bases it may then have that are above the limits. Admins could also require that the new, larger, Airline A divest itself of certain routes to ensure a competitive marketplace (anti-trust/anti-monopoly oversight).
My thought is that Player B (not Airline B) would receive a sum of money from Airline A in exchange for his airline. He could then use those funds, plus whatever he had in the bank prior to the merger, to start up a new airline and begin from scratch. The merger would likely contain a requirement that Player B's new airline not be based too close to his old base and might also contain requirements that Player B's new airline not serve certain cities until a certain date. You wouldn't want to pay a handsome sum to acquire another airline only to have its former owner immediately become cash rich and start competing with you right after the merger.
They key part that would likely be difficult to implement is that we'd need to introduce the concept of a Player's bank account, in addition to the Airline's bank account as funds would flow from Airline A's bank account to Player B's bank account, which Player B would use to start a new Airline B.