How about introduce a non-playing company (doesn't show up on lists, has infinite money) which only manufactures cars using certain rules. (population of town where that company wants cars, ...)
That way it's just the same as introducing a new player, with all the cars being standard vehicles that are just unavailable to everyone except that player.
That neutral player just sets "orders" to go to certain tiles in the city or in other cities (some people like doing long-distance driving)
(OR)
Introduce an option in "configure patches": Enable traffic.
This enables one extra competitor in the game who uses a different AI.
Enable cars for everyone at different dates (just like every other vehicle): They'll have extremely low capacity (2-10) and basically make them completely pointless to use for a normal player. (reduce the capacity, make it 0 even, and make them cost helluva lot)
Make it so that this extra competitor uses a completely different AI that is very simple (as in, not-having-high-ambitions-only-makes-cars simple)
Make it so that AI competitors don't ever use cars. I'm not sure how the AI weights vehicles but maybe weight against these heavily.
Make it so that extra competitor assigns orders to the cars to move to certain tiles, or even certain hidden stations, or existing player stations.
I think only that assign the extra competitor a different AI needs any coding, since we can already have competitors, custom vehicles (using GRFs an' all). That, and the hidden stations (OR) assign orders to coordinate.
Last of all, I don't know much C and I sure as hell won't be able to program. I'm just suggesting a method by which it can be done. Tell me if I said anything stupid
