They should bring more european car racing versions in the game. Like BMW M3 GTR etc. there are many more like those. All the FIA GT cars. And all the cars that are in Codemasters TOCA series. More US old classics. They should look at the 50 Fastest Musclecar list made by Muscle Car Review. All of those 50 cars.
Old Can Am racing cars is a good idea. They were really powerful.
More old rally cars like Audi S1-550hp and 800kg
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And peugeot 205 T16.
There are many supercars and racing cars form 60, 70 and 80-s, made in Europe and USA that deserve the name legends, and the younger generation does not know anything about them. GT4 should bring them alive again. BMW M1 for example. The list is long. And rare older and also new experimental cars, that existed in real life but never made it to production...
PS! Very important is to include more tuning companies and their cars in the game. Tuning is the automotive word of this decade!
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I think they would eagerly like to advertise themselves in GT4. In Japan: HKS, Apexi, Veilside, Blitz etc. Europe: AMG, Brabus, Carlsson, Alpina, Hartge, AC Schnitzer, Koenig, Gemballa, Lotec, Irmscher, all S-series factory Audis etc. USA: Callaway, Saleen, Lingenfelter, Shelby, Hennesey etc.
More...they HAVE TO include dragracing, 1/8, 1/4 mile, one mile challenge-top speed and time. And one drag strip should be in the city at night. Lot of bright lights should cover the track. And you could drag race on the street with 3 other cars- 4 cars in a line, like it is popular today among streetracers. There should be special street drag league. Only in there You could bet Your car- the winner takes it all system
Why I think it should be in the game - streetracing is VERY popular all over the world and no game has not still included it.
In the US there could be always legal problems and lawsuits abou it, but Polyphony has to make this streetracing part of the game this way that no one could directly make a connection to illegal streetracing.