If any of these things show up, it will be big news for the GT franchise:
1. Damage (duh)
2. Weather, time changes, etc.
3. Lamborghini or Porsche
4. Custom liveries, race modifications, editors, online options
And those are just off the top of my head of the most-discussed topics over the past year or so. As I said....even if ONE of these shows up...it will be news, let alone the track and car announcements from the hinted 90+ tracks and 600+ cars. I know a lot of racing sims already have these...but if PD does it, you know it will be grade-A quality, and for the series itself, they would be firsts for certain options.
Also according to Amar's first cryptic post, "all will be revealed no later than E3"...or something of that sort.
I'm 100% confident it will be there....there is no reason it should not be at E3, and as others have said, it would be a HUGE mistake for Sony and PD not to show it.