I liked GT5, I thought it really did a much better job than previous titles of adding a little good, old fashioned fun to the mix of a rather serious racing sim/game. That being said, There are plenty of improvements, namely online, that I can think of.
The Problem: The Performance Points factor was a band-aid to a pretty big problem. In all professional and most amateur racing leagues and events, there are regulations. Regulations on engines size, drive wheels, wheel size, weight, downforce, tires, etc. GT5 did a bare bones job of having these regs. It was simply down to Horsepower, Weight, and Drive, and even then it never seemed fair. There was always 1 car that did better than every other car that weighed 1740kg and had 550hp (Hmm...I can't remember the initials to that car)
The Solution: Well, simply add more. I think the main problem here was, while all cars had numbers assigned to them (For displacement, length, width, etc) they didn't go beyond the 'Car Dealership' information bar. I think GT6 should have these numbers as metadeta on the actual car. This way, you can set a restriction online for 2000cc engines or lower. You could even take this a few steps further and restrict Forced Induction, or even restrict it to V6 or I4 if you wished to. Hell, I think you should even be able to limit by body type: Convertible, Coupe, 4-door, etc.
The Problem: The problem here was, there wasn't actually a filter, or at least one from the 21st Century. Race For Real? What does that even mean? give me some real options.
The Solution: Including what I said up there, have an extensive filter list...no...more extensive than that Polyphony.
Have the option to narrow down the online races to Race Type: Drift, Free Run, and Race
then narrow down to track. then narrow down to regs. If you want to race a bunch of FWD, 2000cc hatchbacks, then you should be able to search for them.
I'm sure you guys can think of more. Discuss!