nice thread here, mate. funnily enough i was thinking this same thing (what id like to see in GT5) a few days ago before i signed up to these forums.
a few things i'd like to see myself in the next Gran Turismo:
-real damage causing real problems; e.g. being able to total the car, slightly offsetting one of the wheels, bursting the radiator... all that stuff.
-more tuning options (basically more customisable engines); twin turbos! not just standard turbo kit stage 1 to 4... more complex stuff. things like twin-charging (using both a supercharger and a turbocharger) would be cool.
-being able to set your own paint schemes for the car, add vinyls, change paint colours once the car is bought etc. and like in one of the old GT series (GT2, i think) where you could 'Race Spec' your car, which lowered it, gave it different wheels, paint, logos, windows, reduced weight and increased power a bit (i think).. that was good, you could make any old car look like a racer. funky.
-being able to do acceleration tests (like 0-60, 0-100, 0-130.. whatever. that'd give us a good idea of how fast our car really is...), turning circle tests and more kinds of tests.
-a wider choice of cars, instead of every single spec of Toyota Supra, Nissan Skyline and Honda NSX ever made...
-making the Gran Turismo F1 car tunable and being able to put different tyres on it.
-being able to further speed races up in B-Spec (3x isn't enough, no way) and when the car pits in, it goes back to the race view screen and carries on fast forwarding the race (instead of leaving it on normal once the car's pitted, for it to go back to the race view screen) - get what i mean there? it's a hard one to explain
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-being able to more drastically reduce weight in your car, for example even changing the material of the whole frame and chassis of the car to a lighter one, even if this conversion is extremely expensive. that'd be good...
-real engine sounds on most of the popular cars; e.g. Jaguar's, Mercedes', Aston Martin's, Ferrari's etc... it doesn't matter about the little cars really. i'd love to hear the V8 roar of the new XKR on GT5... yummy
-larger races, for example 30 cars instead of just 6. that'd be mean...
-real online play against other players
there's loads more ideas i have, i'll post