Actually, with Photomode, they've shown that they can do it if they want to. But a Virtual Garage would
hopefully include only a dozen or so of your favorite cars in a garage, or on a driveway outside your house.
Think about it in a wider perspective. In all the previous GTs, the only prize for winning was a pat on the back, a glorious ending movie (which, honestly, you won't watch more than a few times) and an expensive and fast car... since your garage at the end of the game is already full of expensive and fast cars, maybe this would be a way of rewarding the guys who toughed it out and finished the game. For every percentage completed (10%, 25%, 50%, 75%, 100%), you get a better house, a bigger garage (1 car at 0%, 2 cars at 10%, 4 cars at 25%,...etc... 12+ cars at 100%) and a nicer showcase for the garage. Of course, this garage doesn't limit your TOTAL number of cars, just the number you can display. The rest of your cars go into storage, and you can scroll through them showroom style in the "regular" garage.
An additional bonus would be a nicer neighborhood to "free roam" for some instant car tuning and testing. Be honest, how realistic is it to pay for track time for testing when you can just do a spin around the block?
Even more realistically, you could buy a new "house" for a few million bucks in-game. These "houses" would include the garage showcase, your own personal "free roam" or test area, and possibly bonus race series "local" to your neighborhood/city/state/country.
