Imho an enlargement of the map size to 32×32 km, 64×64 km or bigger ones isn’t impossible, otherwise the map sizes we now have wouldn’t exist.
That would make airplanes an option.
I don’t see, why planes could break the economy.
The plane economy we know nowadays appears around the 1950’s and Planes are very expensive or why do you think you have to pay more in RL to fly over the Atlantic in comparison to crossing it by ship?
The dev’s can easily, with realism as reason, design planes to have very high maintenance costs and have a starting procedure that takes 2 minutes or so, even if you have the airplane line departing immediately, whether or not there is freight.
That would do the job imho.
For ships, sure, the rivers need to be broadened in the generation process, maybe with a marker, up to which point a ship can cross a river.
Maybe the map editor will deliver us island or transoceanic scenarios in the future with coastlines on the edge of the map, maybe some larger islands in the ocean with a few cities.
And also, depending on the ship size, high maintenance costs, comparable to airplanes, so you also use ships only, when you transport huge amounts of freight over great distances.
Also the devs, if they add ships, should think about long distance deliveries of goods, about double the distance, industries have now with their 20 min delivery radius with 40 min delivery time then.
If the ship and planes are faster, they could fill this role.
That wouldn’t ruin the train aspect of the game and reserve Freight ships for the medium, huge containerships and planes for the endgame era.
But, because we have a sandbox map in the game and not some kind of campaing or scenario where you’re forced to do something, plane-haters and ship-haters or even both just can say “Scr** this s***, I stick to trains and trucks only” and theyre fine too and have their “classic” TF.