The title says it all. Me? I guess I'll always have a place in my heart for the Circuit Gilles Villeneuve. I mean it's one of the last great old circuits in F1 racing, and since there are now F1 and NASCAR vehicules in this, it would be awesome to see this. http://bit.ly/9fSKhh 

As long as Cote d'Azure is in it I'm fine. I can't really think of others that aren't already included.  

I would like to see the return of the tahiti road course. GT4 just had the rally version.  
 
Really I would love to see anything that got dropped after GT1/2 

A custom track editor 

the all time great Grand Prix circuits are a must if you want respect from motorsport. Spa, Monza, Suzuka, Monte Carlo, Hockenheim, Silverstone. La Sarthe would be very good too. 
I'm partial to Spa-Francorchamps hopefully with the old bus stop chicane intact!

Rainbow Road

Is Bathurst in there? If not they should make it available for DLC. It is pretty much the de facto race track of Australia. 

Midfield Raceway if it's not on the disc. My fav GT track to race on.

Fujimi Kaido

Red Rock Valley (from GT2). Even though most of my time spent on that track was to win the TVR Tuscan Speed 12 prize car (again & again) to be sold for a ridiculous amount of credits (the Toyota GT-One made it a guaranteed win on that track).

Donington Park