Airplanes on this mapsizes should work, I mean look at Transport Tycoon. The combination of those various types of transport made that game big, I think it should fit Train Fever just as TTD. I think it would be great, but thats just my opinion.
They could add it as DLC, if you don’t like it you won’t have to. Or optional to put planes off before you start a map. Best for of both worlds 😛
Ships should fit perfectly but most rivers should be broader then they now are (from which I saw in the videos, the rivers looked to small for watertransportation).
Helicopters are more easy I think, because there are no map adjustments needed. Just place a Helipad and you good to go.
I hope to see as much variant tranportation methods as possible, it will make the game last much longer and keeps the game intressting for more people.
Maybe a strange idea: Add Taxi’s !!! How fun would that be?, and realistic too: here in the Netherlands it’s very common, Trainstations are very crowded with Taxi’s waiting for passengers !!!