In short, I would like to see customisations go as far as PD has the time and ability to handle.
In GT5, I get this impression that you're just clicking buttons and forking over credits to make your car go faster. This should not be the case. For example, when reducing the weight of the car, what exactly is being removed? What CAN be removed from, say, a Lotus Elise, or a Mazda RX-7, or a NSX-R? All of which were no compromise sports cars to begin with. I am a moron and I love to keep my cars street legal and useable on an everyday basis, hence I appreciate my air conditioners and stereos. Do THOSE get tossed as well when reducing weight? It irks me that I don't know, especially when the air con vents are still present in the cockpit view. And, yeah, maybe they replaced some parts with lighter ones, but then the question I have to ask is: how does it affect the car's weight distribution and centre of gravity?
If we are given the option to replace our hoods and windows with lighter alternatives, I don't see why this practice can't be expanded to such an extent that it eliminates and replaces the "Weight Reduction" option entirely. It makes you feel more involved, more personal, with your car. Or, for the casual gamers, the "Weight Reduction" options might be retained as a "quick option" that automatically selects parts to be tossed/ replaced.
What about the Turbo systems? Instead of saying "X Range Turbo", why don't we have genuine aftermarket turbochargers instead, like we have aftermarket wheels? I remember back in GT1 and/or 2 where we also had to install the intercooler on our own, instead of having it spoonfed to us.
I would also love for aftermarket tyres, not just aftermarket rims. The tyres you buy in game for your car don't really behave any differently: they just have less grip. The way they "bite", slip, and everything else feels exactly the same, which I find a really lame concept.
But what I want most of all is more aero kits. Standard cars suffer especially in this area: they usually only have rear wings that are black and "tacked on". Contrast this to the spoilers you can add for cars like the Ferrari Enzo or the 458 Italia: they fit the car, come with matching paint, and for the Italia's case, they even continue with the stripes of the car on the spoiler! What I'm trying to say is, I want more of the aero kits on the level of Ferrari in-game. And let's not forget often overlooked cars like the Mazda RX-8 here. The RX-8 even comes with a spoiler fresh out of the factory if you buy the R3 edition, yet all you get is the fugly, obscene, highly obnoxious black one from GT Auto. No OEM spoiler, no Mazdaspeed spoilers, no aftermarket aero kits like RE Amemiya's, no nothing.