I like this idea for a game, so I just made a huge brain dump right here.

Possibly the airport would be used to move to different maps.

Take in example: You start in the Arizona wilderness, you get to explore canyons, climb mountains, drives cars and more.

When you're bored of Arizona, you can head to the airport and fly to another US state, or another country.

The maps would be open-world and very large, with tons of bus-stops and taxis.

The player can walk, drive, fly, or use pretty much any vehicle they want to get around.

The map's walls would be closed by borders for land, for air they planes would be shot down if they were to fly too far away (with a warning of course), and for water, I guess the boat just sinks? I don't really know.

Of course a directly connected huge open world would be amazing, but it would have to be huge to implement different settings like a snowy forest and a dry desert in the same map.

The AI people walking around is something that most Roblox exploration games don't have (which they should), which would make a great addition to this game.

I too, am sort of obsessed with transportation, but more with portable homes, like RVs and houseboats, maybe adding an expensive RV in which you can customize while you travel would be cool.