Hoping for more Group C and LMP cars in the future. There are already some great ones in the game, but most are obviously standard. Some others I think are worthy
Porsche 962 (probably impossible, but perhaps the Dauer version might not be)
Mazda 767B
Mazda RX-792P
Audi R18
And of course the absolutely brutal, legendary Sauber C9 needs to be made premium. What a machine that is. Also, I think that some of the lesser C2 cars of the group C era are worth noting, machines from Spice and such would be a cool addition.
Something about closed cockpit prototypes just gets me going. There is something elemental there. It must be the fighter-jet like cockpit. I love driving the closed cockpit race cars at night in GT5, seeing how these legendary endurance racers appear in the dark, where a considerable portion of their time is spent. Very evocative things GT5 is capable of.