First things first. I 100,000% believe that we will not see motorcycles in GT5. Gran Turismo is not Midnight Club. Why do you think PD would kill their own franchise deal? Tourist Trophy is the motorcycle love PD will grant GT fans will get to enjoy. Just because GTHD had motorcycles doesn't mean that GT5 will feature them. GTHD and VGT were complete tech demo deals. GTHD featured all those different vehicles because they details were of high detail. Why do you think you seen motorcycles and that Tank Car? It was to show off the capabilities of the PS3. Those bikes and "Special Cars" in GT4 are very detailed. PD would be stupid to get rid of Tourist Trophy in only its first game. I'd love to see some sort of Tourist Trophy 2 where about 24 or so motorcycles can be on track at once. So I doubt a game featuring CARS would feature motorcycles as well. I'm sorry. I just don't see that happening.
I don't care much for stock cars, so I'd hate to see them. It would be good for most gamers because there would be more types of cars to use. It would be interesting to test the limits of these cars in action. There would have to be more ovals to make for authentic action. I wouldn't want to see NASCAR in this game except for maybe some classic race cars.
Formula One cars are already in GT even if they are fantasy models. There are some quality race tracks in the GT series which you can enjoy F1 racing. It would be a nice deal if some more F1 models were featured, especially since Jensen Button recently won for Honda. It's a nice deal for Japan to have some quality talent take advantage of a Japanese make's machine.
Drag racing cars were featured in GT2. They would be pointless in road racing even though they were featured in GT2 with the HKS Drag cars. One is a 180SX drag car and a 4WD Skyline R33 drag car. Very little chance we'll see John Force's Mustang in action, not even if you use the Vegas Drag Strip or whatever. I SERIOUSLY doubt this.
Coupe/Stuff Not Said... not sure what the heck you're mentioning here.
Tuner cars would be the most possible, but on a conditional basis. Tuned cars modified with more capable cars would be a wonderful oppurtunity to go racing. But don't think we'll see show cars which make for horrible driving. Same goes for outlandish body kits and such.
Future cars? I SERIOUSLY doubt that. Unless you're absolutely think the car of the future will be like the agile Nike One. Unless you're H.G. Wells, I seriously doubt we'll see anything futuristic.
Jet cars? Unless you're talking jet drag cars, maybe. Jet cars are worthless for road racing. They'd probably drink up fuel like water.
I'm not an online gamer, so the downloadable cars (while realistic) is something that is completely realistic.
As for what I'd like to see in this list, I'm completely at a loss. No definite answer for me. So have no favorite answer for this deal.