After reacqauinting myself with TXR0 with my saved game, the fictional #13 GT500 Skyline just doesn't feel the same anymore. You can tune Jintei's Skyline to 700-something HPs (and oh yes, you don't get to have the cheesy electric bolt effects around you. I think TXR3 probably ENCOURAGES that you race it a lot and a lot and a lot until you get the engine change for that car. It's the same with TXR0. A lot of miles = new engine and parts for your car.
As far as the West Corvette goes, I think that's one of the most awesome cars in the game, even if you don't like American cars. It didn't have the same grunt of the car in the intro movie. I'm not a fan of the C4 Corvette, though. But racing it (after having a battle speed of 126+ mph, I finally challenged him in my Ford GT.
I know it is a real car (after looking at the link), I think it would be cool if the West Corvette was a tuned C5. If not that, why not the 2003 C5R that ran Le Mans in 2003. All you have to do is add the West Corvette graphics.
If you want to REALLY dig into groups, why not race against the tuner car teams. You know, Blitz, Yashio Factory's pink machines, RE Amemiya (should make for impressive battles using rotary power, and how about facing some of TOP SECRET's cars? I know the theme is shady underground racing, but to face real tuner teams and earning their stickers will really test your skill. And who knows? Maybe the next big boss will either race:
-A GT500 NSX
-A GT500 Supra
-A Viper GTS-R
...or I'm going to go out on a limb. Maybe the next Speed King or White Charisma might race:
-A Nissan R390 GT1
(I know you been waiting for this)
-a Toyota GT-One
You want to make challenges more intense? Better drivers, bigger teams, and bigger grudge matches.