The variety of races we saw in GT4 should be in GT6 along with the ability to qualify.
Official series events like the Super GT, the FIA GT3, the VLN, NASCAR and the pseudo-LeMans events should be present.
One make races like the Audi Tourist Trophy in GT4 should make a comeback especially for models that have a wide variety of versions present within Gran Turismo (like the Mustang of which we have 5 different premium models and 4 standard models if you count the Cougar XR-7), the Corvette (7 Premium and 7 standards), the GT-Rs, the Evos, the AMGs and et cetera.
Polyphony had the right idea around GT4 in regards to events and likely pared down on events due to lack of room.