Everyone knows the feeling, i am sure: you lease a plane on the UM, and it really is convincing you, and you want to buy it... but no, it is not leased from AI, it is from another player. So you contact that player and agree on a price... but no, you cant just buy the plane. You first have to cancel the lease, and then the other player has to put it back on the market etc. etc. etc.
How about expanding the functionality that is already given with the AI leases, to be able to buy out the leased aircraft. The owner would have to activate a checkbox (like currently for "lease can be prolonged", but instead it is "aircraft can be bought out"). So after that is activated, the lessor can purchase the aircraft directly, instead of having to return it and disrupting the services.