I know this would take some time and development, though it's something I thought of while reading your post and it seems like an idea that would attract some attention and keep members coming back.
You could hire a coder to design, develop and produce a system for your auto-forum which would allow members to buy, sell and trade "virtual" cars. Members would earn points through posting, replies and uploading photos (possibly?) of which could be spent on the buying of these "virtual" cars.
Members could then sell cars they've bought, trade them with other members and possibly race them against other members of your forum and challenge other members to a race.
From the design aspect, micro cars could be designed and would be displayed in their profile under a new section. The profile would state whether the car is for sale or not, the amount of wins the car has had and so forth.
If you wanted to get really technical, you could find a good developer and integrate random car damage into the system that would take place during and after a race.
You could also integrate car theft, allowing member to attempt to steal another members car/ride.
Just an idea to get you moving, more could easily be said