Quite simply, while it would be potentially interesting, I think any kind of mass transit/public transportation system (buses, planes, trains, etc.) is beyond the capabilities for TS4.
But I absolutely agree that we need cars and motorcycles, as well as improved boats (most importantly, boats that can seat multiple sims).
I have also long asked for a Traveler/World Traveler trait along with a conflicting "Homebody" trait. Ideally, the Traveler trait would increase the likelihood that you would see those sims out in worlds other than their home neighborhood, while the Homebody trait would make it less likely that you would see them in other worlds (but more likely that you would see them in the neighborhood park or out for an evening stroll).