Congratulations on the world series win.
Today I won that championship in a Mazda RX-7 with the engine from a 787b!
I really enjoyed some missions, but did find some of them annoying or worse. Worth a look through the list.
More races should have unlocked for you. I really enjoy picking a car from the garage that I haven't driven, taking it stock to suitable races. Then, if I like it, working it up through the ranks to as high as it can go. The Lexus LC500 has probably been the biggest surprise, going all the way to WTC800 and winning them very enjoyably.