Definitely! But GT5 doesn't need more cars, per se. It needs a wider, more comprehensive range of cars, IMO. Not just old but middle-aged and new, too.
This has been a GT5 development mystery to me from the get go (as has been stated ad nauseam):
Why have 1000+ cars, a "museum" of car history collectibles, and NOT have a more complete range of historical cars? Where is the Model T for example? The Alfa Romeo Alfetta 158? The Pontiac Bonneville Special? The Lotus 25? You could go on and on!
Surely PD doesn't believe the SKYLINE to be one of history's most legendary of vehicles.
Granted, I don't know how many Nordschleife races I'd do with a Model T but it's probably about the same amount I'd do with a Schwimmwagen. Of all the many confounding miss-steps PD have made in their otherwise brilliant game (stunted menu design, half-baked customization options, under-developed course maker, etc, etc, etc, aside) this one bothers me the most. They seem to have taken a step in this direction and then abandoned it for more GTRs and Miatas.
Diversity would seem like a no-brainer. Instead with GT5, arguably one of the PS3's and Sony's most profitable and revered franchises, Polyphony Digital focuses an insane amount of effort and detail on some of the least interesting cars.
When a game as beloved as GT does so many things right in terms of graphics and physics it still feels like such a disappointing missed opportunity that the rest of it's framework (some of it quite innovative in it's initial concept) is so strangely skewed and unfinished.
This is coming from someone who has played the game religiously since launch, and to be clear, I love the game and still laud PD for their successes. I just hope, for GT6's sake, PD can learn from their mistakes, from their fans, and from their rivals, too.
(rant over!

)