Here are my suggestions. Some have been mentioned by others of course, but since GT5 is still being worked on ...
1) Better AI - tunable, so we could race against better drivers than ourselves in order to better ourselves
2) Better car selection - as diverse as possible - if we can have 30 Skylines, we should have every Aston Martin, MG, Mustang, or every Porsche if the license can be obtained
3) Allow better tire selection, ie, based on real tire selections from different manufacturers (I know, more licensed objects!)
4) Give us a paint shop, car graphics (stripes, numbers, etc), including no-paint like the MBs of 1950s
5) Real time car damage, including tires going down, flats, blow outs, bad brakes, and body damage
6) As a race car uses fuel, it should get slightly faster, just like in real life
7) Handling should change over different surfaces in the same race
8) The presence of oil, gas and tire rubber should change the handling on the racing surface
9) Weather changes, including different weather around a course, just like in real life
10) Extreme lighting changes, ie, day and night, track lighting, and of course head and tail lights (including reverse lights on production cars)
11) Ability to save replays on USB flash drives or other removable media
12) More tracks goes without saying
13) Real rally races, including co-drivers to do timing, etc on real multi-stage courses
14) On-line play in all modes, that way one could get a quick race in arcade mode against your buds
15) On-line play with championships, races scheduled by the racers so all races don't have to be done in one session
16) True class racing, ie, like in the SCCA, or multi-class racing like in ALMS, Rolex, Le Mans, etc
17) In conjunction with Logitecha and others - better wheels and controls - including a clutch that could be used with cars that actually have a clutch - paddle shifters should only work with cars that actually have paddle shifters available! Shifting should be able to be more like real shifting, including missing shifts, skipping gears as you shift, etc. If a car only has a sequential gear box, that is what should be available
I'm sure there's more to ask for, but this is all I can think of at the moment.
All I can say - is bring it on
