This is a bit random, but I just thought of it now and I feel it would be really fun to be able to create a private or custom world where you can invite certain players to join that world. It would allow you to compete with friends and set up really cool scenarios. Or, if you have no friends, you could set up a world with no traffic restrictions and try to set up an airline flying literally everywhere.
I know it's random, but it's just an idea