I agree with your suggestions theuros… I suggested much the same. I think having airports and shipping ports treated as industries/destinations, rather than as “stations” for your own planes and ships, would make a lot of sense within the scope of Train Fever. I’m all for that as a future development.
From a pure flavour point of view, it would be really nice to have small propeller aircraft, helicopters, tourist boats/sailing boats, etc, etc… but I fear that most of those things would only really make sense as flavour elements – they wouldn’t meaningfully contribute to a mass-transit or cargo transport network. Commuter boats and canal barges could have some meaningful utility, but even then I think it would be fairly niche, and so it would be understandable if it wasn’t an important focus for the developers.