Dynamic weather and day/night shifts!
A lot of those options are dependant on other things.
For instance, "proper" damage is pointless without aero modelling to go along with it, otherwise it's just useless cosmetics.
I also think the tyre physics would likely need redone when we have dynamic track temperatures and wet weather effects acting on them. Maybe not, but you never know.
But then the day/night shifts will require a graphic update before it'll work properly, so should that come first?
Now you see the reason Scawen doesn't publish his plans