A good suggestion in my opinion, not sure it has been already mentioned:
If I fly LHR-SIN-SYD I can fill my planes with passengers interested in LHR-SYD, LHR-SIN AND SIN-SYD. In the game if I wanna take advantage of the kangaroo route I can not "use" the demand for the intermediate legs, correct? It would be interesting to possibly be able to sell tickets both for the full flights AND the legs of a flight with a technical stop.
Anyway it's an awesome simulation, well done!