We already have 70+ cars and 6 locales. They were awfully quick to give us some of the newest content to the GT franchise. Daytona, London, a re-paved Eiger, a new HSR. Even Fuji and Suzuka are significantly improved over GT4. Then there's the cars. We have the C6 Z06, Viper SRT10, The new M3, then R35 GT-R. Oh yeah, I almost forgot to mention we have most of the current lineup from Modena.
If they're willing to give us all of that in Prologue (now that we know that prologue isn't going to be expanding as much as we might have originally thought), I for one am excited for what they're potentially holding back on us.
Consider this... everything we have now, except for HSR and Fuji, will be outdated by the time Mo5 hits. There's supposed to be a longer version of Eiger and London. Suzuka is due to reopen next month after major renovations to update it to host the 2010 Japanese F1 GP. Daytona has been updated over what we currently have. Every car we have should be replaced or updated by then as well. ZR1, Viper ACR, the F430 replacement, complete with the same transmission as the Cali, GT-R V spec. There so much more to come from cars we already have.
You're just getting anxious.