I want a game where the mechanics are so detailed, that one can switch out the spark plugs, play with their gaps, change engines, fine tune the engines, such as the ability to play with it's timing, have tons of gear ratio options and have the ability to change tire pressures, ride height etc...just as we can in real life.
I doubt the next NFS series will have much improvement in that area though.
NFSW was decent in that area, and Nascar 14 was pretty damn good in that area, but it seems that the racing industry cares more about how real the cars look, than on how real the mechanics of them are, compared to real life.
Who cares how real the tires look, if we can't even change their individual air pressures?