Oh this is going to be a long list lol:
- pretty standard to want more cars, more premium (less or no standard), less duplication of models (3 or 4 mx5 is enough). What I would really like to see is a cockpit view for all cars!
- More tracks is always good. The return of a few GT oldies (Apricot hill, Seattle etc) would be nice, but more real world tracks would be better. Spa is a must IMO (and judging from what I've read on GTP as well as other sites, is in very high demand...good chance we will see this, perhaps even in a patch). Some of the non-Tilke designed F1 tracks would be sick (Tilke sucks IMO...should stick to designing hotels and grandstands, leave actual track design to someone else lol...although both turkey and china gp tracks are growing on me). Catalunya, hungaroring (sp?), Malaysia, and brazil would be great

Test track needs to make a triumphant return! Even though it ain't my thing, 1/4 and 1 mile drag tracks...how they did not include one is beyond me. Could be programmed by a 6 year old and take up less disc space than any one of the seemingly hundreds of yaris or mx5 variants.
- more in depth tuning / modifying. GT5 seems to have got very 'easy mode' in this aspect, probably to attract a wider audience. I remember first picking up the original GT and being overwhelmed by the tuning - would be nice to get that feeling again. As previous posters have stated, I would like the ability to take any stock road car, mod the crap out of it, and race it 'on the weekends'. More in depth control and options concerning engine internals, more control over engine aspiration options, engine swaps (some thought would be required for this. Probably easier to program in only 'real world' swaps, such as putting an rb20 in an s15 for example. I would think a little more than engine/engine bay dimensions would play a roll in unlimited swaps, and would need to be accounted for to keep things realistic), drivetrain swaps etc etc, it's all been said before lol.
- more visual customization! Whether people like it or not, this game (along with almost every other video game out there) is going to become more online oriented, which is going to make visual customization more desired. Paint system needs to be overhauled, tokens or whatever the are called need to go. I do somewhat like the idea of unlocking rare colours along the way (or perhaps trick finishes like pearl and chameleon), but all colours in gloss, matte, and metallic should be available from the start, or have a custom colour creator. Preview before painting is a must! Even though some hate it, full control of vinyls and stickers as well. If I put brembo brakes or an hks turbo on my car, I want to be able to put the respective sticker on too.
- vehicle specific visual mods. Only example I can think of at the moment is say I have a Sti Sooby, I want to be able to chose between the fog lights or sti covers...stuff like that. On the subject of lights, upgrades here would be welcome, as I always feel I'm blind as a bat driving at night in gt5, even with tv settings adjusted.
- Full control over all wheel/tire dimensions (sidewall thickness, stretch, rim size, offset, dish, etc etc), all of which have large effects on performance...tire pressure too, can't believe GT missed that one and Forza didn't. More control over painting rims would be nice too, as well as calipers.
- I would like to see far more aero upgrades, at least to the level of Forza, but preferably more to the level of Import Tuner Challenge for X360. Would be nice to see real world body kits and aero parts (and for the record, I despise rice, I'm talking only about functional aero parts that increase downforce, reduce drag, reduce weight, and increase cooling). Fender flares and wheel arches are always welcome lol.
- More cars with RM option, and perhaps tied into this, widebody kits.
- more inclusion of tuners, both full cars and parts. I like how spoon, amuse, hks etc all have models in the game...let's see more!! Top Secret, Jun, Tech Art, Alpina, Brabus, Koinegg...little rusty on the usdm tuners, but I know theyre out there, more the merrier! GT seems to have licenses to use the names of dozens and dozens if not hundreds of tuning companies (used mainly for track decoration/sponsorship, as well as some vehicle sponsorship), how about we actually get to use all the stuff these companies make (I want to put pink Sti suspension components on my sooby lol, not just 'suspension mod lvl 3').
- IMHO, forget about NASCAR, carting, and F1. Focus more on perfecting production vehicles and GT race cars (rally cars I can tolerate, and tbh, Tarmac rallying is one of my favorite things to do in game). I have nothing against these other forms of motorsport (except NASCAR, can't stand that garbage), but I think they're better off left to their own releases. No one in the gaming industry has yet to correctly model the open suspension of an F1 car (reason why suspension is hidden behind winglets in cockpit view, and why the cockpit view looks distorted)...codemasters got close with their latest f1 installment, but still need work. And tbh, id rather not see a single penny of the $60 Im going to pay for gt6 go towards that greedy sloth bernie ecclestone, cause you know he'd make kaz give up his left *** to get the licensing. Every second person on this board seems to complain about NASCAR handling, and for good reason...it sucks in GT5. No I dont think it should be easy (show me what's easy about driving a 3000kg, 800hp car at 200mph and I'll show you what makes NASCAR entertaining to watch), but it should be somewhat realistic, and PD failed to make it that way. The drafting is a joke, bump drafting even more so...the garbage AI doesn't help either I suppose. My point is I get what people mean when they say 'don't cut stuff, only add', but I would rather see PD focus their time, effort, and disc space on perfecting GT road racing, and leave these other forms of motorsport alone (you wouldn't want your next madden game to start including FIFA mini games in it just because they are both forms of football would you?)
-More intuitive and user friendly menus!!! The main GT mode menu, although it looks great, gives me fits sometimes when I'm navigating and the cursor decides to go random directions, same with the tuning menu. Would be nice to see units beside all tuning specs where applicable. Would also be wonderful to be able to switch between imperial and metric without having to exit to the main menu (or just get rid of imperial all together lol). The track selection menu online is for the birds...everyone was high the day that got programmed. Others have already proposed solutions, most of which were already in previous GT games.
- better AI, goes without saying
- if all the improvements concerning upgrading (performance and visual) get implemented, an online dealership to sell/buy/trade cars (including set-ups) would be great.
- a real track creator, not a broken randomizer