Hood view
Skids
Smarter, way smarter AI. Less braking, more sharp reflexes, if there is an opening i want to see them go for it, especially when the car in front is slower. They should take some risks, like braking late to overtake, or taking the outside in a curve, even if they could lose the backend. Right now an AI overtaking looks more like a space shuttle reentering the atmosphere than anything else.
Custom ingame ui, like in nfs porsche, set the map, dials, lap time, position, etc. where you want, and the option to disable them individualy. A scale option wouldn't hurt too, UI is way too big (GT5P) in 1080p, even 720p, i can read my lap time from a mile away, it doesn't need to cover half the screen, it can, but it doesn't have to.
More than one display style for ui elements, like 1/4 1/2 or full map, analog/digital speedo, distance and/or time for ahead and behind label, etc. And if i want the actual car dials instead of the generic ones, make it happen.
Virtual dials and mirror in cockpit view.
Ability to turn the dash lights on, or at least have them on at all time.
Something should be done about car lights not being on in daytime, in older cars you do have to turn them on, but now it's automatic, it doesn't feel realistic to see a 2010 car with its head and tail lights off.
PS: this feature might be a regional thing, i can only speak for north america
Adjustable cockpit seat position
When i win a car i'd like to choose the color.
In series events, i don't mind winning a random car the first time, but let me chose wich car i receive the next time i win that event. Do i really need to win five NSXs in a row before finally getting the Supra.
In splitscreen multiplayer:
-Cockpit view
-Random car selection, sometimes i don't know wich car to drive, let the game select it. "UP+Start". (could be a single player and online feature too)
-Let both players select their car at the same time, GT5P almost had it.
Skids, mostly hood view, but skids too.
An event creator, you choose track(s), opponent(s), restriction(s), etc. and the game calcute credits based on difficulty/length factors. Maybe let the game put a prize car in there once in a while, like a 25% chance. Gimme a randomize option too.
Make every career events available to race online, no car prize tho
A mirror, a reverse, and a reverse mirror for every track.
A damage engine on par with codemasters' toca/grid and dirt series, especially dirt 2, it's like carmageddon 2 with better graphics

. I don't care about rewinds, there are better things to learn from codemasters, great things.
Full game install
That's all i can think of now
I'd like to end by saying that i haven't had a PS2 for the last 10 years, this post is mainly inspired by my experience of GT5P and some old memories of GT1-2-3, features might already be in GT4 and follow in GT5, i would not know.