So I have a bit more thought out answer to this. I don’t mean to revive an old thread but i thin kthis is a good idea.
I think, one way to add more challenges mid game is to have contracts for organizations like emergency services (police, fire, medical) along with possibly even military.
Each vehicle has a set of required features. Some that are available for all cars (like cruise control, bull bar), while other required features are only for the contract (roof lights, paint job, internal computers, radio transmitter). This feature list could change each time the contract is requested. The other AIs can bid on the contract as well, with lowest bidder receiving the contract. The contract will list the body style, requested features, price for each vehicle sold. The price per vehicle is set by the player when bidding on the contract. Players can not see other bids until he submits his own. The player has 1 day to submit a bid.
Part of the contract can be paid up front, and the rest on completion. Failure to do so will require paying a fee greater than the upfront installment. It could require
x amount of vehicles shipped in
y amount of days. No vehicle with these gov’t features is allowed be shipped without a signed contract.