Truck definitively ( the road and a race version would be great). It would fill a large hole in the simulation universe. I wouldn't buy S3 for just a new track and a new car but if it was a truck I think it would be worth it!
Ohterwise IMO S3 should have:
buggy
500 - 700 Bhp supercar
300 bhp rally car
Muscle car
Kart ( not supermarket)
Stockcar (nascar)
Old F1 without downforce
Legend GT
LMP
700 Bhp GT1
heavy road car: 2000 kg (~BMW, Mercedes)