I hope I won't repeat an already proposed suggestion, though I search a bit on this forum...BTW, my suggestion was to keep what the player owns and already built when going from a scenario mission to another.
e.g., when starting demo mission 2, the player will start exactly in the situation he/she was when finishing demo mission 1 (when pressing "next mission", when starting a mission from scratch the game would do as it already does). So the player would have the satisfaction of having everything done by himself.
Of course, there would be exceptions, e.g. between level 1 and 2 of the second campaign, since the player goes "200kms away" with the car he/she just crafted.
What do you think ?