Oh man, got so many ideas I want in there.....
Fuel gauge (gas refills in race) Temp gauge (be able to overheat, say if you push a weak or old car uphill too hard)
Have the gauges of the actual car there ( no more generic tach and speedo, although they look nice) IF I have a Silvia, I wanna see Silvia gauges.
Transmission accuracy. I mean, most newer model Mercedes Benz cars don't come with Manual. So why not make that true? You should only be able to pick a transmission if the car comes with both in real life
Different types of blow-off valves. On different levels of tuning your car, you hear a different BOV. Or you could just choose which BOV you'd buy.
Pick the size of your rims. I wanted bigger rims on my Hachi-Roku (AE86), but in GT3 you took whatever size the car came off the lot in. pick their color, be able to powder coat them, or chrome them. Those watanabes better be in there....
Carbon Fiber. I want CF hoods. CF trunklids. CF wings. CF over fenders. Hell, be able to make your car's body out of carbon fiber. You should be able to paint the CF too if you don't want a whole carbon fiber colored car.
Rim selection. Bigger selection, and when you buy them, they should be automatically sorted when they go into your collection. Sort them by spokes, color, weight, etc.
Front clip exchages. Like I should be able to make a Silvia an Sil-Eighty, or make it a One-Via. Or Levin fron bumper with Trueno rear and body.
Blinker color changes. I wanna be able to change those nasty orange taillight blinkers/reflectors for some nice smoked black ones.
Exhuast changes. Give us the power to change the look, color, and sound of our mufflers. When you choose which one you want, you should be able to listen to what it would sound like on your specific car. Have real exhaust brands like 5Zigen, and Fujitsubo.
Choose between Supercharger and turbo. It'd be nice to have a choice whether a want a boosted or blown car.
Tuned rom. I know they have it, but you should be able to set at which RPM the fuel cut-off comes in. (I love to over rev sometimes to listen to the engine

)
Backfire. I want backfire. Big, loud, flashy, cool backfire. Like, twin belches of flame when you downshift.
Special engines. Like the AE101 TRD for the AE86, Engine swaps, RB26DETT (skyline engine) into a 180SX.
Rotary engine sounds. I know it sounds real already, but if I can hear that sexy whine of a rotary engine I swear I'll squeal like a little girl.
custom color. I wanna make a color sheme I want, on a specific car. After I buy my car, I can still pick my color.
Online options. Be able to have team logos. Be able to choose limitations for the race. Such as nothing over 250hp or no super soft tires.
More AI cars. I want more cars out on the track. Smarter ones too.
Mountain passes. I want Pikes peak in there. I want some sort of touge there. What's the fun in a AE86 without a mt. pass?
Weather. I know it's in there, but I'll still say it. I want to pick the season, time of day, weather, damp ( post rain) or dry (no rain) roads, snow roads. Weather changes on the fly, meaning during a race.
Better looking garage. I wanna be able to walk around my babies and ogle at their sexiness. I want to be able to pick what KIND of garage. Whether an indoor carousel, or parking lot garage.
More pronounced engine sounds. I just bought my 360 Modena, I better be able to hear it well, or I'll cry

. Be able to set each sound differently. Like stronger engine sound and weaker music or tire sounds.
Cooler car wash. I wanna SEE my sweetie gettin drenched and soaped down (sound nasty huh?

) I wanna see my car get dirty too.
Tire marks. I bought a turbo, I know I can spin those Nike's on my cars paws but I wanna see two, or four (4WD) tire marks to proudly show everyone else.
Body kits. NO RICER KITS. Some nice, subtle changes like the TRD wing on the AE86, or a slightly lower side skirts.
Wide body. If I got an RX7, I want that Mazdaspeed wide body on there. Nothing like, 80 feet wide, but give my car some muscular bulges on the side, make it look tougher, more intimidating.
Better tire effects. I love hearin tires scream. Make more sounds.
Better music. GT3 was okay, but most the songs I turned off. Rap is DEFINITELY NOT race music. Good as normal music, but not RACE music. I do NOT wanna hear Xzhibit's songs while drivin on the Nurb'. Some nice Eurobeat (initial D

), techno, rock (but no NOFX or Korn), somethin fast paced that makes ya wanna run that car to it's absolute limits.
Evolution III. I want that Evo there. It's my favorite, and once again I'll cry if it doesn't make it in.
Custom decals. If I have a drift/ race team on GT4, I want my decal to be proudly shown. Place it where I want, how big I want, what color, etc.
Debadging. I wanna take the name badges off my car, it makes it look cleaner. Sometimes you can't tell if it's the fast or slow version of a car. Anybody say "sleeper"?
Realistic TCS ASM placement. Since when could you buy a Trueno with TCS and ASM? Make it standard for cars that come with it, make it an upgrade for cars without it. Buy it for like 1 grand each. Or for beginners, free.
Interchangeable stock rims. If I see rims on a stock car that I want on MY car, I should be able to buy/swap/switch them once I get the other car.
Lights on/off. I say the select button should toggle your headlights/tailights on off. Whether in the day or not, I love seeing my pop-up headlights. I wanna be able to change headlight colors too. Be able to change headlight looks/eyebrows too.
Reverse lights. When I back up, I wanna see those light up. How hard could that be to put it in?
Interior views. I know, it'll take up too much space on the DVD-ROM so it ain't goin it, not in GT4, at least. But sometimes, I would love to be inside that dream car.
Showrooms. I want the car shops to be kinda like a showroom. A huge dealer warehouse(s) . Test drives.....maybe. I might be in there forver with those Ferraris in the start of the game
Torque steer. Front drive cars have it without an LSD, so I want it in there. It'll make it more fun to drive that 80's Accord with that logitech wheel.
Sorry if the list is really long. I had so many ideas burst out at once. I still have more, when they come to mind I'll list em