FIRST AND FOREMOST: To include\activate the POINT TO POINT feature seen in early pre-release screens of Course Maker.
I think what is limiting its potential is the fact that all layouts have to loop and so must find a way to reconnect with the last sector, in a point to point layout the road can go just about anywhere, and even end in another section of the map, increasing the number of configurations.
I REALLY like that PD have put more "environmental" touches to each Theme, with "roads" and paths across the landscape. More of this, but in Toscana Tarmac and MtAso in particular, more signs of LIFE to make it more like a road than a road-like circuit in an isolated area, more trees in the "no mans land" areas.
TRUE 180 degree HAIRPINS, this is a must when creating "tarmac rally" style stages, as well as K-turns (90degree turns at a cross road).
A (minus) -10 (aka one single lane) ROAD WIDTH @ Toscana, I like -5 but i try to drive in one lane to make it like a tighter country road you would see in WRC events.
NEW THEMES: English Countryside.
Have a look at Victory By Design, the nice little estate trail they use is perfect, tight, winding, wrapping around a lake with a forest section, very simple but very beautiful.