Citta Di Aria..I'd welcome its return anyday!
Hopefully called "Assisi" this time around.
As far as fantasy tracks go, the only ones that interest me are Midfield and El Capitain, and even then, I could go without them (but I know there's people who would really appreciate the addition of some classic fantasy tracks). But I've never seen a game developer go to such lengths to create realistic street circuits - I've personally verified the accuracy of Città d'Aria / Assisi.
I believe Polyphony, unlike a certain American developer that deemed them "unsuccessful", may release one or several track DLCs during the life of GT6. They did that with GT5, too, adding Spa and the SSRX. They could do it again.