How about one huge collaborative game?
I know, iRacing, GT5, TOCA, rFactor [2?], Richard Burns Rally, and, well, basically every single "good game" made, all have different programming codes, but, GT5 has lots of cars. iRacing has decent physics, cautions, flags, and dedicated start procedures that feel real. Richard Burns Rally has what every rally game should have. TOCA has an interesting career mode. Forza has good car customization.
But, each of those has each others' flaws:
iRacing:
-daunting Photoshop work taking, generally, hours
OR simple three colour liveries
-Never, ever changing tracks
-No atmosphere (no rain, no clouds moving, no debris, no oil spills...)
GT5:
-no livery editor, just a general paint car/rims
-limited amount of depth in racing (cautions)
-Artificial [lack of] intelligence that can't keep up with fast drivers and gives up easy
-rally stages that don't do much (not many fun turns), with only somewhat decent rally physics
Forza:
-Absolutely no changes to the track surface, EVER.
-Every lap is hot-lap.
-"banned from servers" for livery editor = nudity, violence, hatred, etc.
-General lunatic community of non-brakers, rage quitters, and porno-makers
TOCA:
-Doesn't graphically compare to GT5 anymore
-physics aren't as good as iRacing's physics
But, with GT5 surfaces and renderings (shiny, metallic, occasionally very 3D,) iRacing physics, GT5 rallying/endurance/night-to-day transitions/weather, GT5 pit strategy, Forza livery editor, and TOCA AI/career mode, you've got the best game, for either PC or console.
GT5 proves that the graphics are possible.
iRacing, rFactor, and a few other PC games proves that the physics are possible.
TOCA proved that good AI was possible.
Need I continue?
All we want for GT5, or GT6, I imagine, is:
-Moar of GT5 (cars, tracks,)
-livery editors
-Further adjustable AI (meaning that they can be set to go faster/slower)
-true racing environments (every lap is different)
-physics (especially wet physics) that actually work
-Weather that happens instantly (downpours, puddles, rivers), and isn't some hurricane that pours all of the way across the Nürburgring.
-Well-implemented (and optional!!) cautions, to aid in breaking the monotony, and giving us a break from the improved/faster AI.
