I know that this has been suggested previously, but now that Hoover doesn't have a stock market at all, I thought it would be good time to bring this up again.
New feature: Ability to form joint venture subsidiries with another airlines.
For example, Airline A and Airline B invest 4 milAS$ and 3 milAS$ together in a joint venture airline. The new subsidiary then has 7 mil AS$ to start with. Control and legal home country would be with the majority owner (in this case Airline A). Airlines would be able to later adjust their ownership by selling stock to each other at book value.eg. Control could be handed over to another partner by a sale of stock.
To prevent cheating, following restrictions would be in place. In order to be able to invest in a joint venture, the investing airline has to have a minimum book value say 20 mil AS$. And at any time the airline wants to make new investments, it's total investments in Joint Ventures cannot exceed say 20% of it's own book value.
Joint ventures would have to pay dividends to the investors.
The legimate use cases for this feature are Joint Regional Airlines, Joint Cargo Airlines, Joint Terminal investments etc.