Lots of great suggestions up there, some not so great but if it all goes in we can ignore the bits we don't want to use (Am I the only one that thinks Photo mode is a waste of space and time, but at least I can choose not to use it).
George Morley
Looking at some screenhots of Gran Turismo HD, it looks like there will be 4+ cars in 1 rally race, thats crap, there should be a set time to beat in rallies.
Rallies, yes, but Rallycross, like Cathedral Rocks I & II are a mixture of tarmac & dirt with up to 10 600hp+ Fiestas, Escorts, 911s etc.
My bugbear since GT1 is to be able to take any car in your garage, at pretty much any state of tune and not get a challenging race easily. Something like the synthesizer in GT2 (might have been called something else) but with more scalable difficulty than that did. Maybe using A-spec points by putting in the number of points you want to have eg 200 and it gives you equivalent opposition.💡
Also like to see proper power and torque curves back again, the ones in GT4 don't cut the mustard.
Also, why have such variations on power depending on the screen you are in. If I have a car with 200hp, why does it show up as 185hp in the garage, 190hp in the settings screen, 205hp in the replay etc. OK, published power figures in the showroom can be different, we all know about the gentlemans agreement thing in Japan and the reality.

But I want to see what my car puts out. This one is probably a bug though so I'm going easy.
More stats - Yes I'm a stat freak

. Fastest lap per course per car. overall lap records, highest speeds etc. Most successful cars by wins and win percentage.
Finally, a better way of penalising bad driving, or fix the 5sec penalty thing. I think its the Swiss Alps Normal course or something where the opposition immediately goes right on starting, whacks into you and you get a penalty for flooring it and going straight ahead.

A proper damage model could be the answer but has been discussed ad nauseum everywhere, but until the AI can properly distinguish blame in the event of a coming together then a penalty system is not going to work very well.
Almost forgot, all tracks and [whisper it in this thread] cars from all previous GTs, and to be able to race on them in proper races. And I mean all, not the GT4 version of all old tracks (wheres Red Rock, Tahiti (not the maze), Smokey Mountain etc)
EDIT: oooo just had a flash of inspiration 💡 💡 💡 💡 Similar to one of the other ideas posted, but how about that you get invited to take part in races that are in the right power range for your type of your car, and also your ability level based on licences held and gold licences passed. Saves having too scope out the races in advance to make sure they aren't too hard/easy which is what I waste a lot of time doing.