I have a LOT of over the top ideas so just write the most important futures I want in GT5 down:
1. In car-view, aka cockpit view or dashboard camera.I wouldn’t have a problem if PD cant make cockpit views for all cars, IMO I don’t need 700 cars, give me 500 or 400 with cockpit view and I am fine; in car view is a must have if PD still wants to make the ''real driving simulator''. An adjustable seat would be also a great feature, and there should be a pair of hands at the driving wheel, it feels very odd to have a driving wheel which spins without hands on it.
2. Better AI What can I say about that? The AI of GT is very bad, I want intelligent race driver opponents which overtake only if its save, I don’t like the kamikaze style of GT drivers. Having safety-minded drivers which take care of their cars and recognize you is a extremely needed feature which must be in GT5. Its the biggest drawback of the GT-series, and its not so hard to do that they cant make it.
3. Engine sound I am not an expert and I don’t know what PD did false with the GT4 engine sounds, but they are too weak and the lack of the ''power'' sound really kills the atmosphere. Its also odd that some engines sound almost the same even if there is a difference of 3 litres and 4 cylinders! IMO its not extremely important to record all original car sounds, its much more important that engine sounds are kinda logical. I also hope PD improves the turbo sound, they don’t sound all the same, there should be a difference between a high powered race car engine turbo which propels the air to almost sonic speed and the every day turbo of a supra. Just making the turbos ''hiss'' louder doesn’t solve the problem.
4. Night / Day change, if PD cannot make weather they should at least make night / day changes of all tracks, especially on tracks with 24 or 12 hours enduros. It would also add a lot more atmosphere to the track. I imagine a feature which allows you to adjust the time you want to start the race. Wouldn’t it be great to drive at the Ring at the crack of dawn? PD would also have to add real car lights for this feature!
5. Weather As discussed in another thread, I would like to have weather in GT5. Nothing extravagant , just rain, wind, and a changing skybox.
Wind should affect cars downforce or aearodynamic, in a bad way, rain should turn into heavy mist at high speed straights and behind cars and should blur the view in the cockpit, and the wiper should work like in real live too. Rain tires and intermediate tires would be a necessity.
6. Track atmosphere PD made a lot of pretty tracks, but a lot of them suffer of a lack of atmosphere. As example lighting is one problem, IMO some tracks have a shiny weather but the lighting is like on a cold rainy day. But that’s not all, I wish PD would add ( weather independent) puddles, random wet spots on the tarmac or little misty places to the tracks to give them a bit more character. Or at Laguna Seca some sand on the track would be cool; adding spectators to the Nürburgring would also be a nice addition, mabye a few tents with spectators and smoke of spectators having a BBQ at the Karussell.
Heat waves is also a great idea, which I read somewhere in this thread.👍
7. Full pit-stop control and yellow flag / safety car periods, real racing rules Its an important part of racing in real live racing, the AI should make some crashes with their cars and the safety car should pop out like in real life. It would make and advantage, you could go into the pits during a safety car period without losing too much time.
Full pit stop control is a must in GT5, drivers aren’t stupid, we can drive into the pits with speed limit and stop at our pit crews.
Another rule feature I want in GT5 is when the AI crashes you very hard they should be DQ or they should make an extra pit stop. I think I don’t need to mention that I want the whole pit lane crowded with pit crew members.
8. Real races Also a big drawback of GT3 and GT4, GT2 had a lot of races where race cars of ONE CLASS drove for the first place. Its just stupid to have a 40 year old LM race car racing against a LMGTP Audi R8 and a 88 group-C car.
Please PD, make GT5 races like in real life.
9. Body kits and engine overhaul Engine wear does not make sence if we cannot do anything to get the power back. In real life race cars get an engine overhaul after almost each race to prevent the engine of losing power.
Body kits should be als included in GT5, I don’t need to mention that its unreal to buy a rear wing which can also increase the front downforce.
There are a lot of body parts which would make a highly tuned cars better looking and a lot faster.
PD started with a rear wing, they should continue with front and rear fins, a set of different front spoilers, NACA ducts and intakes which create a better airflow to the turbos to add power, and maybe even underbody ducts and diffusors to increase downforce dramatically.
Of course these features would consume a LOT of programming and coding time so I am pretty sure we don’t get that kind of car customizing.
10. Race modification In GT2 it was a great future, I want it back!
11. Porsches Its time for Porsches, really it is! RUF's are nice but they are not Porsches. And no, I am not satisfied with 3 or four different 911 versions, I really like to see the great Porsche race cars.
I expect to see the Porsche 917, 917-30 Langheck and Kurzheck,the 935 Moby Dick, the 936, the Rothmans Porsche 956, some different versions of the 962 like the IMSA Busby Miller 962 (gold & white with low custom aerofoil) or the Löwenbräu 962with its unique extra spoiler at its front nose. They are legends and they were giants in their time.
That’s the most important stuff PD should add!
Each GT game, it becomes harder and harder to breakout, so they might work on that again.
Or I stink at searching?
because everybody should have a car that "PD" people set up. I mean you know everyone's car settings are the same just as are their driving styles. 
Also let you tune your car to a more realistic HP. I always wanted a really fast Subaru 360, GT3 let you tune your cars for higher HP, why did they lower it? The Suzuki Alto Works had 238HP in GT3, but now it's limited to 125HP in GT4. And of course Engine Swaps. Imagine a Subaru 360 with a RUF CTR2 engine!! 
