Simple : play with chronology but the years don't increase and the vehicles avaible are always the same witheout future vehicles.
I think this extension must not be very difficult to make and should be cool to create scenarios ;)
Quick thought: a different approach could be to play without timeline (earlier name for this), but allow the scenario to pick the objects (vehicles, way tiles, ...) from the pak-set, e.g. by naming them, or by specifying the timeframe from which the current ones would be taken.