There won't be an option where players can hostily take over other airlines, as that would be very unfair for the player who has been bought out if he couldn't control his airline (= play in the game) anymore - or that other player would say what he'd had to do and manage. So that would have to be sorted out somehow ...
Generally the stock exchange would be cool thing to have but it can be rather complex, and plus it's not a core feature of the airline business. As I will try to model the standard features first.
Currently when airline goes bankcrupt all their aircraft are put on sale to public markets.