When building or extending terminals there should be a checkbox for auto-booking the new capacity for yourself upon completion.
The upcoming change regarding cancellation of flights from overbooked terminals will bring with it the need to more often extend terminals, and many of them. Keeping track of all extensions is cumbersome and error-prone, as there is no ingame way to do this. Automatic booking will eliminate the need to keep track of the extensions as well as the need to re-visit each station after completion. => A LOT less manual work with a simple checkbox.
Good to have both - terminals and operations - in one company for this feature.
Going with this even further, there could be "regular = fixed contracts" and "preferred = flexible contracts" between terminal operator and the airline(s). Regular (fixed) contracts would be limited to the actual contracted capacity. Preferred (flexible) contracts would be flexible and could be INCREASED beyond the initial fixed contracted capacity, as the airline grows. The contract would be "augmented" as the airline grows, to the maximum extent of available terminal capacity. So the terminal operator could chose what contract to offer - maybe fixed contracts to 3rd party airlines, and flexible contract to own airlines within holding and alliance partners. Once the terminal capacity is booked to 90%, terminal operator could receive a notification message that he is approaching capacity limit, and he would receive the "capacity limit" notification as well once the full capacity has been reached/booked. In case the terminal is fully booked, flexible contract customer airline would receive "terminal over-subsribed" message (as it does in present time) and capacity could not be augmented any further until the terminal operator extends the capacity. The contract customer airline could decide whether to run the risk of cancellation and have the terminal oversubscribed, or have it assigned to AS default handling. Once the terminal operator extends the terminal capacity, the terminal customer with flexible contract would reassign those flight to the private terminal, which would then in turn trigger contract capacity augmentation as per the flexible contract.