I would like to see my heroes' outfits within the game:
Ayrton Senna (Toleman 1984, JPS Lotus 1985 and 1986, Camel Lotus 1987, McLaren 1988-1993). The helmets and the suits have subtle changes or major ones throughout the years, so PD better get 'em right;
Alain Prost (Renault years, McLaren years, Ferrari and Williams years);
Nigel Mansell (Lotus, Williams, Ferrari, Williams *91-92* and McLaren *95*)
Apart from these three, pretty much anything Formula-1 related will satisfy me, specially vintage stuff (over modern ones) from the 70s, 80s and 90s, like Jean Alesi, Heinz-Harald Frentzen, Emerson Fittipaldi, Gerhard Berger, Riccardo Patrese, Jackie Stewart, Niki Lauda, Kimi Raikonnen, Mika Hakkinen, Damon Hill, Michele Alboreto, René Arnoux, Stefan Johansson, Stefan Bellof, Johnny Herbert, Ivan Capelli, Maurício Gugelmin, Karl Wendlinger, J.J. Lehto, Thierry Boutsen, Érik Comas, Gianni Morbidelli, Éric Bernard, Bertrand Gachot, Alessandro Zanardi, David Coulthard, etc...
I do, however, highly doubt any of these will ever happen in Gran Turismo.
I would also like to be able to wear manufacturers outfits (like the ones we see the pit-stop team wearing, or the driver in arcade mode wearing, depending on the brand of the car you're driving). And I would like also to be able to change outfits whenever during pre-race when on online lobbies or lounges.
And I think an outfits shop and a paintchip one would make more sense than having to rely on seasonals for one (or trading), and having to buy cars for the other (like paying 300k on a car you already have just for its color's paintchip).
And yes, I would like female avatars... I just don't know if the programmers would be able to make every single piece of outfit already available in the game to fit the new body-shape. Maybe a tick box for every single piece of outfits for gender selection ? Well, PD was always about minute detail (which always putted GT above everything), and they've missed on this one, 'cause they knew they had female fans prior to GT5's release, and not only that, there are professional female drivers out there for quite a long time now in IRL, NASCAR and others, and there were a few in F-1 as well throughout the history.