The only way next generation Premium models can be more detailed is by adding functional detail as engine bay, doors and hoods that can open, chassis detail, detachable panels when damaging the car, etc. However existing models will be perfectly usable, in no way looking "old-gen".
In case processing power in the next-gen Playstation allows, all they need to do to increase detail in order to make them even nicer looking (if you notice, in certain cases GT5 models can look "low poly" at a close distance) is bump up subdivision surface one step, then optimize the resulting 3D mesh and generate/tweak the added levels of detail needed for the game to smoothly work. With previous PS2 3D models they couldn't do that because they were made in a different way as I mentioned. And also, increase texture resolution (for interiors/dashboard...).
PS4 GPU is going to have DX11.
These models needs to be accurate as they are now. The only thing that can make them look better is just by adding polygons to the current models.
Then in-game by adding tessellation (which makes disappear those low poly curves) and more post processing effects.
Colour palette and lighting does the rest and you can have a realistic looking games.
Of course then comes textures that should be high quality for the tracks and environment to make it all looks more realistic and compound. 3D trees should do the work on the tracks too.
And more animated objects as those wheel protection walls around the tracks. Tracks in GT5 looks realistic but "death" at the same time.
Cars will be the same for GT6.
Compare GT5 cars to these videos from rendering:
By the way...Ray Tracing is the future of gaming.
Check this out everyone: http://raytracey.blogspot.co.nz/
As you can see there this is rendering but Ray Tracing can be used for real-time games as in the latest video. (i have that and a game similar to Portal 2 just to prove that this can be done with more GPU power than current generation...that´s why this could be and will be the future...almost true to life graphics)
And there you can compare GT5 cars with the 2 videos above...we don´t need better models at all if they are accurate. But they could have better textures inside for the interior. (take a look at pCARS textures and details for example)
As a last thing i wanted to show something that i find too. Ray tracing using 3 PS3´s back in 2007.