- New suspension, tyre and aero model. I doubt that would have been possible to patch into GT5. At best they might have been able to tweak the old model a bit, but that's about it.
- New graphics engine - definitely not possible to patch into the old game.
- Way shorter loading times, especially when navigating the menu. In GT5 you had to wait about a minute every time you wanted to do something. Loading the friends list to gift a car took like ten minutes. Navigating the menu was like navigating the Bermuda Triangle - you never knew where you'd end up and if you weren't careful you could end up in the wrong place and had to wait a minute before you could go back to the menu.
- A structure that's easy to work with. Primarily this benefits the developer, but in the long run it will also benefit the user. Patching this into the old game would be like having an old pair of trousers with holes in them, throwing them away, buying a new pair of paints and call it "patched".
- I'm not sure about this, but to me it seems like GT6 has an improved online connectivity. When playing on a wireless connection in GT5, I used to get thrown out of lobbies most of the time. In GT6 I haven't been disconnected once so far. Of course, it could just be that something has changed with my connection since the last time I tried to play wireless...
What would have been possible to bring to GT5 is:
That's nothing more than a bit of new DLC.
If you miss the missing features from GT5, you can still play GT5. It's exactly like a spec 3 would have been, except that it's missing some cars and some tracks.
Well the course maker is brand new, so there's the explanation for that. It takes time to develop. Things like shuffle racing, I don't know if they're missing because it has to be re-coded for GT6, or if they didn't like it for some reason or if they simply forgot about it... B-spec is coming, apparently in a new and improved (hopefully) form. Community features, whatever they are, should be just around the corner.
It's not only additions, but also changes to the old models. There are always things to improve. The suspension model is a great leap forward from GT5, but still it's not perfect. Tweaking the models would probably be possible in GT5, but if the model is bad to begin with it's often better to start from scratch and build a new one.
And I don't think it's a spec 3, because it's not an addon to an old platform. It's a new platform waiting for things to be added to it. It's a quite natural process in a world where games keep developing past their launch date, that the game that's at the end of its life cycle will be bigger than the game that has just started.
GT5 is old, sick and in pain and it has to be put down. But there's a new puppy in town. It's the circle of life.