Honestly, I don't think that's the right question to ask. There will NEVER be enough tracks, especially with the trend that PD has set for itself with the monthly free update. Even if we get a thousand tracks, people will start complaining when PD stops releasing these monthly, free updates. That's my guess, anyway.
What I am happy with is how PD keeps giving us all these spectacular updates, for free! Each month's substantial update has enough content for any other publisher to charge for them, and PD just marches to the beat of their own drum, for the better or worse. What I find lacking in the course list is the lack of variety in the track list thus far; they're all wide, paved, high speed courses meant to facilitate Gr. 3 and 4 cars. If you bring any of the many production cars added later into the game, they'll put you to sleep. Try bringing an FC RX-7 to Barthurst, and hopefully you'll feel how the car absolutely comes alive in the narrow, bumpy corners full of elevation changes. A lot of these street cars, without Gr. 3 and 4 renditions, feel very lost and out of place. I wish we had more narrow, technical tracks like Tsukuba where these cars can get to shine. I mean, what the heck else are you gonna do with an S660 and a Mini? Bring them to St. Crock or whatever it's called?
Personally, I have a love-hate relationship with the fictional tracks in GTS. They're all immaculately designed to challenge a racing car and a driver's technique, full of small nuances that can spell the difference between a win and a loss. They're a lot of fun, for sure, and I genuinely believe them to be the best fictional tracks in the genre, even moreso than fan favourites like Deep Forest and Grand Valley. It's just... why would I want to learn some ambiguous fictional tracks, when that knowledge is non-transferrable to other games and even real life? I got a heck of a kick learning Red Bull Ring because I play the game that touts itself as a "Real Driving Simulator" for the realism.
tl;dr, I want more real life circuits, and more smaller, narrower, technical tracks for the slower production cars.