Everything is very nice and clean, except the vertical grass going over white lines and kerbs.
There is one problem, and that is not the lack of objects IMO, ofcourse there is a lack of objects, but thats not exactly only that. I think the problem is the monotony. And it is not a short track. Trees are almost giving out a pattern. Also they are equaly dense and high everywhere, there aren't any which would stand out from the rest. I checked google streetview of real location, it is monotonic but much less lol There are some areas which are almost completely cut out, with just a few trees left, then very dense at few places, then less dense, yonger and older forest areas....
Those concrete blocks are very monotonic, I guess you will do what Lilski suggested.
I'd suggest to use more colorful tires, or the material which covers them, just to have more color, because there is huge lack of colors. Think of more ways to win colors.
I think that grass still lacks color variation too, but I have no idea... It probably would be much too hard to have more variety and still have matching with the flat terrain texture.