The most obvious thing that could make GS transportation infrastructure better

[kryat]

In some ways, I think Civ 6 has almost become a different game through the introduction of tunnels, railroads, and canals; turning from a civilization simulation game to a ditch digging simulation game. And I love it! There’s few things in the game more satisfying to me than being able to move a freshly built tank from one end of your empire to the other in a single turn.

We’re missing one thing though, that would reflect modern life better and make an existing district far more powerful: international air travel. I think you should automatically be able to airlift to any ally’s airport, and be able to negotiate “open borders” with anyone else to be able to do this. This alone would make a world of difference to late game mobility, but you could even get fancy and expand it to allow basing aircraft at military ally bases, and also have a separate “open borders” type negotiation with other allies for the same feature.

 

[Tomice]

Sounds awesome IMHO - late game mobility is essential to keep the late game interesting. Without mobility, you can't do much against an AI that's about to win peacefully in faraway lands.
It may be even more important the other way round: If you are about to win, an AI that can assemble forces within a few turns would be able to keep you challanged.

 

[gallowsCalibrator]

This is a good call! Would make air infrastructure better, which is always good. Something I'd also really like to see is being able to airlift to- but maybe not from?- airstrips. Would make them more used too!