The GT-series always needs more old, older, classic or youngtimer type of cars in my opinion but most of all I'd like to see some true vintage machines build between the 2 world wars.
Vintage Bugattis, Bentleys, Mercs, etc. or once famous brands of that period which don't exist anymore.
It's a period not only overlooked or ignored by PD (apart from including the Auto-Union and Model-T in GT4) but by almost all developers of driving/racing games, at least I can't name a game where they're included.
To be honest, it's not a period I'm normally particularly interested in but it would offer a driving experience currently not available, and a true challenge mastering these pre-war monsters on skinny tyres.
I'll always remember the moment when I was at an Italian-themed carshow/trackday with all sorts of classic and modern Ferrari, Maserati, Lamborghini cars including things like several F40's, an early Countach and the latest Murcielagos when all of a sudden an 'uninvited guest' came through a tunnel leading to the infield and produced a very loud and glorious sound upstaging all the other cars, everyone present looked to see what it was.
Turned out to be an early thirties straight-eight Bugatti sportscar, not the fastest, most exclusive or unique version Bugatti ever made but even a 'regular' Bugatti roadcar upstaged all other exotics by simply blipping the throttle in a small tunnel.