I liked the Rail road tycoon version where you could as an "CEO" buy personal shares and you could also buy for the company, maybe even a bit more complex system would be like Capitalism II but i dont think that level of detail will be put to the stockmarket alone as its a pretty small part of the game (or big for some like me, but there are other games to satisfy my needs

wall street raider etc.)
But if the devs are planning a stockmarket i hope the do something between RRT and Capitalism II, also it would be a nice option if there would be some regulations/fines if you try to manipulate the market. But that might be hard to add, but it would add alot to the stockmarket as in Capitalism II if i wanted i could become a multi-billionaire in about 15 min just by manipulating the market.. So while thats nice its not so challenging after that

Anyways i just realized this was in the Developer question forum but i'll post it anyways since it kind of fit the thread

EDIT: Another idea, buying shares of oil companies or aluminium or whatever raw material and you would get free parts when building a car or they would just produce cash (for some time, when resources are out you would have to buy shares in another one). These companies should be "automated" as in you cant improve them or run them in anyway they would just give you benefits/money if you own them/own a part of them.
EDIT 2: That could also give you huge strategic options as you could low-ball the market if you wanted with so cheap cars that if your competitor would try to match your price with the same car they would lose money.