For years now, one of the dreams of many players has been Infrastructure Sharing and City Owned Airports. There have been several attempts to do this, and I recall on a multiplayer game I played once, there were indeed some city owned airports that had been added before the map was loaded.
Gamescripts already have APIs that allow the building of pretty much anything the player can, including airports, docks, roads, railways, and canals. However, except for roads (which will build as town owned) these must be done within the context of a currently extent company.
I propose that in addition to the current companies, the Gamescripts be allowed to build things as the neutral company. The same one which provides the dock/heliport for Oil Rigs. This would allow gamescripts to be written that construct the airports when a city gets big enough that all players could use.
The game already handles the neutral docks and heliports of oil rigs, so this should not be that big a code change. As I mentioned before, I once played on a multiplayer map that had airports that had been set to this company (presumably by hacking the save) and they worked fine (or appeared to, not sure what they had done server side to get that working). The companies still had to build one airport of their own, of course.
Bus stations, especially the drive through ones, would also be something that would benefit from being built by gamescripts (which would allow building airports with a bus rank, for those occasions the script screws up and puts it just a little too far from the city) under the neutral company.
A further suggestion would be building of neutral vehicles such as eye candy cars, replacing the AIs that do this for you. Though I'm not sure whether this will be possible due to costs and the like.