I do sort of wonder how I'd personally handle the birthday sign-in bonus in a future GT game, especially since the next game may or may not necessarily have a car for every potential birth-year to gift like we saw in GT5. I want to say that I'd gift a set amount of credits (which in turn would have its value dependent on the in-game economy of that respective installment), as that could be genuinely useful for many players (whether it gets used for buying cars or upgrades) and there'd be no worries about obtaining duplicate vehicles if you happened to sign into the game for two (or more) of your birthdays, one year apart. It does sound a bit boring, however.
So alternatively, maybe the game could gift the player a sizable amount of credits, but also a voucher for a certain vehicle class. I personally really like the idea of vouchers as prizes, as it lets you select a car [potentially either a specific model, or a certain class, manufacturer, drivetrain, etc.] & color later-on if you wish, and without the pressure of settling on a color immediately like in GT6.