A few suggestions of my own...

More realistic engine starting. Where you have to press and hold the starter button to operate the starter, and hold until the engine fires. How long it takes would depend on engine temperatures, outside temperature (thus affecting engine temperatures if it's a cold start), bus model (and thus the age of the bus), level of maintenance (see later suggestion), and so on... A full cold start on an older bus that isn't up-to-date with its maintenance could be hard to start, while any bus on a warm day would start quickly.
Maintenance of spawned buses (separate from repairs). Buses up-to-date in maintenance would be reliable and easy-to-start. As the maintenance level degrades, the bus might start becoming gradually more unreliable (random stalls where it can be restarted, different bus components may start to individually fail, to an outright engine stall/breakdown where the bus has to be fixed or towed to the garage for repairs). New buses being spawned would start with a random maintenance level, from a user-configurable range. This feature could even have the option of being disabled for those who don't want it.
Depots able to have indoor parking, with functional garage doors.