I think that an option to go to lighter weight parts is also a nice consideration. The weight differences mattered a lot in Tokyo Xtreme Racer 3. Your basic metal choice was FRP (don't know what that stands for), urethane, and dry carbon. To me, I think one option you can use when using a racing mod is the option of carbon fiber parts in natural color or colored (I think natural color in TXR3 meant lesser weight than painted). The weight differences will be enhanced. But to me, it would be a good option.
I'm sick of hearing "rice" even though I mentioned that word in an earlier thread. I don't think any body kits that contain parts to "ugly up" a car will be in GT4. I think GT4 is more on racing, not on tacky kits. "Hey John, I thought you liked art. You snitching on us or something?" What I'm saying is that... well, I tend to agree with Solid Lifters. I think it would be cool to come up with such body kits, but only if they don't seriously take away from the car's looks. An example would be the Honda Accord sedan in GT2. The racing mod had a nice DTM look to it, I actually liked it. Made it look more powerful. It's a weird idea to give a wagon some new bodykits, but for those who attempt such tuning, I'm not hating. Anyhow, I don't think that any (and I'm going to impersonate a "rice rocket" hater) "already ugly cars made uglier by ugly body kits" will be in Gran Turismo 4. I think if a body kit made for GT4 will improve its looks and actually compliment high-horsepower tuning, then I say bring on the body kits. But if you cannot have body kits, then carbon fiber body parts are a great consideration.
I still stay true to the ENGINE SWAP option. You may want to upgrade to a stronger engine, a lighter engine, or maybe a more enduring engine, while making as much horsepower as possible. If a 1968 Mustang will be in GT4, I think the badboys of muscle car horsepower here in America will want to invest in crate motors which provide more horsepower to dominate in racing. I mean, how do you horsepower lovers feel about swapping engines? Question is, where in the Tune Shop will there be space to perform an engine change? I'd say that if it's engine related, it should go under the "Engines" category. And like I said in my "Menu and Presentation Considerations" topic, I think when you go to "engines, you can choose between N/A and Turbo engine parts. A supercharger would work for non-turbo engines and should be a nice option for engine tuning.
But in the world of tuning, there are limiltless modifications to tickle one's fancy. So, keep thinking of cool tuning stuff to be implemented into GT4.