I think it needs more pickup trucks, perferrably some that are worth driving. Out of the three or four they did stick in, only one is driveable due to being 4WD. Also, more vehicle-specific parts. If one buys a truck, lift kits and swampers should be on the menu, but for a muscle car drag slicks and 4-links and the like.
Three damage modes, none, limited and full. On full, if you slam something, it immitates what the real life car would do in that impact, so taking a Dodge Ram 160MPH into the ass of a Mazda 737B doin 30 going into a hairpin would end up as a pile of shattered carbon fiber and one mid-engined, shortnosed pickup truck with in-door steering, not a 737B in orbit and the dodge at the perfect speed to drift the corner. Fun tho that is, it's a bit arcadish. If the vehicle is repairable, on appropriate models, airbags would also have to be replaced to meet race regs, and even still the car would never handle right again. It also activates mechanical failure. Persistant overrevving, lacking oil changes and constant braketorquing can caust the engine to overheat and eventually grenade, complete with internals flying around. It would also model regular wear, such as brake linings wearing away, rotors warping, clutches starting to slip, steering flop and bad suspension joints showing up if one's willing to venture that far into the vehicle's life. On limited, impact damage is off, but mechanical failure is still on.
Better fuel mileage modeling. Having a 726HP pickup truck get better gas mileage than a 550HP civic is unreal, especially when the truck has a steady diet of race gas, nitrous and 160MPH speeds.
Aero should be tweaked. An option should switch between current the current method of modeling it and an super realistic aero modeling. On that setting, any stock pickup truck should be undrivably squirrely by 120, and if left unmodded, will blow over if the driver manages 130.
Add better cars. We dont need 300 cars, where only about 100 of them are worth driving. lose all those 20-70HP crap cars that waste disk space and add things like Lambos, Porches, Ferrarris and the such. We also need the vehicles from the styles of racing listed below. Who knows, taking a NASCAR-spec stock car to the 'Ring and playing with the GT1 cars might prove interesting...
More vehicle-specific races. Most trucks should end up in an offroad-style race. I usually use my Ram for pummeling people into the German countryside at the 'ring. Pointless, really, but fun. A 1970 Challenger a GTP car does not make, yet mine's pretty quick and snappy. That car is better suited to a drag race environment, and what self-respecting auto nut takes a Skyline for a snow drift? There should also be no restrictions on the cars you may use in things like practice and family cup. I have always wanted to take my Model T to the 'ring in photo drive mode. Also, toss in a few American styles, such as NASCAR, NHRAMonster trucks and other, less well known race styles. Things like demo derby, lawnmower races and belt sander drags are not subject to inclusion.
Better physics. Take a full-size truck into a 2G turn and it's going to begin a perfect immitation of it's tires. They're beyond topheavy, and drifting one from grass onto asphalt should result in a snap traction roll. If damage is turned on full, the wheels on that side should take on about 45º of positive camber and never roll again. I love racing trucks, I have all the pickups and SUVs GT4 has to offer, but I find the physics disappointing. I managed to flip my Ram once, and it nearly ruined my PS2. Even if you can lower one enough to keep it on it's feet, it's never going to be able to pull 2Gs. It's going to break one end loose, usually the rear if it's being driven right, tho they'll favor a push if one just swerves or brakes, then swerves. That part's fairly realistic. My older Pajero just begs to flip, you can see it's hefty hind end leaning heavily. It will easily hike an inside tire. It should be on it's side. This should also be toggleable, so people like me can take their pickups into family cups and just play around with them for the fun of it. Those that strictly drive GTP cars probably wouldnt even notice the anti-flip being removed, since these cars are so low slung and hard to flip anyways.
Install an option to allow electronic aids only on vehicles which posess them in real life. I find it odd, yet extremely required for racing, that I can fit a 1970 Plymouth Superbird with TCS and ASM. It drives wonderfully with them on, but because it was made in 1970, technically it shouldnt be there. This option should be disableable tho, some of us acually want b-spec bob to be able to control our muscle cars, or maybe even ourselves controlling these tire-eating beasts. FWDs are also incredibly horrid about burnouts, and they tend to push like bulldozers while doing so.
Better tire physics. A car with R5 tires and only 350HP should not be able to powder both tires in sixth gear at a dead stop. My '88 accord has that much with NOS on and will easily do so. All motor, on only about 250HP, it will powder both through fourth. It will still chirp them with TCS on 7. This is obscenely overexagerated(never could spell that, bear with me). True, a 350HP FWD should be able to ruin them into second, by third they should be firmly planted, especially with an LSD in. One should be hard-pressed to get a 4WD to let go on dry pavement period, yet if I remove my LSDs, my Ram will easily roast the inside front and rear well into third gear when I sauce it out of a corner. An 1800KG vehicle shouldnt be able to do that with only 750FT LBs of torque. I also wanna see better smoke. Someone braketorquing a muscle car with R5s in the back should generate a smoke cloud visible for the next four laps regardless of the track. Someone drifting a car on S2 or stickier should generate a similar smoke cloud, tho more dispersed. Also, rubber sliding like that should leave some serious marks that stay there for a few weeks.
Engine physics. Engines are not indestructible. Someone who never changes the oil and always lets it bounce off the rev limiter is going to grenade it, and I'd like to see this in the game. I'd also like to see it removed from cars that dont acually have one, or are too old to have one. They barely even had engines in 1886, let alone rev limiters. There should also be a temp gauge that, if pushed too high, will cause the engine to start to misfire and lose power, and if pushed even higher, cause the engine to weld itself in place. This should cause a spinout and the need to replace the engine, possibly other drivetrain parts so shocked by the lockup. This should be a disableable option.
More high-end mods. If one is willing to blow enough credits, FF cars should be able to be changed to FR or 4WD(even more expensive), with a differant type of engine installed. I've seen online real Ford Focuses that have recieved a bolt-in RWD conversion. You supply smallblock ford V8, money and labor, they supply parts. This should be doable in the game. Also, some kits will affect handling differantly than others. The V8 kit I mentioned is said to not change handling or balance all that much, tho not having driven one, I have no personal experiance. Muscle cars that came with smallblocks should be able to have their current engine bored, stroked or both, or completely replaced alltogether. There should also be a used engine market that stems from this option, which is a great place for people with grenaded mills to get their car mobile again.
Better trans modeling. An automatic should not be a game-shifted manual, instead it should perfectly model the way the one originally in that car is. It should model the slip, the engine zinging to just under stall speed and staying there till things get moving fast enough. Also, if a car originally only had an auto, a small charge will have to be paid to have it converted. The same applies to the other way. Shift kits for both should also be available. Some cars, such as the lone buick in GT4, would be victim to this charge. If a car was available in both standard and auto, and is purchased new, you choose the trans you get at time of purchase and dont have to pay to have it converted. The auto and manuals would also vary in speeds, ratios and tuneablilty. They'd also have differant default tuning, for example auto's shift points VS manual's gearing.
Better car purchasing. One should be able to choose various driveline options and perf packages available to the buyer of the car in real life. One could, for example, choose a I4, V6, hybrid or SI option for a 2006 Civic. Luxury packages will not be modeled, but those with performance updates will show with only the name and perf parts.
Differant method of procuring higher-end perf parts, as well as parts for those makes without an in-house tuner, by the means of a third-party parts shop. They would also carry things like displacement UP, engine swap and the like.
MP3 support, so you can play YOUR music through the ingame radio. Also, if you purchase the right parts for the cars, you can change the way it sounds.
And finally, a better AI and matching system. I took my Dodge Ram to the 'Ring and maxed the hardness and the worst pest I had to knock into orbit was a low-end Zonda. 'Twas a breeze, I had the power and binders to slip past him and the mass to absorb the inevitable bump-n-rum he'd try to get back infront of me. The game acually had the nerve to put me against an S600 once, I think on 5. Anyone who's ever driven one knows they're so pathetic it's hilarious. I was in first by the 5th turn. Not enough of a challenge, so I went to circuit de la sarthe and had teh same problem...