Personally I think LFS already has a good balance of cars. Although some more selection in the lower end of the spectrum would be great, some turbo charged variant of a front wheel drive hatchback that has a lot more peaky torque would be a great addition, something hard to master, but easy to drive.
I have to agree with some others about a V8 as well, something great for straight line but terrible round the bends, would make for interesting racing.
Nascar may be an idea, although thats kinda a the GT variant of a V8.
One alternative addition to LFS is a Pace car, AI controlled to do the warmup laps and a rolling start.
I personally think LFS should stay away from F1 cars, LFS could stand the risk of becoming yet another standard racing sim rather than something different. There are tonnes of F1 racers, but not many cross racers, and thats what makes LFS interesting.