Disclaimer: This is only about vehicles as world objects, not as driveable entities.
In my opinion, one way to further improve the game world would be to add a few new vehicle types.
First and foremost, I’m thinking of busses, ambulances and fire engines, because we have bus stops, rescue stations and fire stations in the game, so if new vehicles were added, I think it would be natural to add those three.
Also, since Östertörn is a rural region with a lot of wilderness, a jeep-like offroad car would make sense, maybe with a civilian, military and police/forrest ranger version, would make sense.