Well, let's say if Forza, Colin McRae and Richard Burns Rally, Toca, Project Gotham, Tokyo Extreme, Need For Speed, Live For Speed and rFactor didn't exist, we might not see half the things Polyphony is putting into GT5.
But remember that many of the things that people want are based not just on games but real life. Things like extensive tuning and customization, drift and drag, rally racing, car painting and so on crept into racing games from real life. Also, people were drifting in Gran Turismo as well as other games before PGR and NFSU. We've been harping on the return of race modification because it appeared first in Gran Turismo 2.
Kaz will occasionally mention other games, and you know his team have to have an eye on Forza, since Forza basically took the GT framework and spiced it up a bit to become the only real competition. To be sure you can see Kaz including things from Forza that have been proven popular with gamers like the paint shop, body kit and name brand customization features. But not only do these teams borrow from each other - Photo Mode in Forza 2 for example, but Kaz has said from the beginning in 1997 that he wanted to recreate the racing world in a game as much as was possible on the PS hardware. The one game which pioneered innovation in a racing game to encompass as much of the autosport world as possible was Gran Turismo.
This time, he has some serious Playstation hardware, so I'm expecting something equally ambitious in GT5.