Are Turn 10 gonna be using a brand new engine for Forza 6?
They cut corners across the board to achieve 1080p/60 for Forza 5.
Doubtful a new engine will be used but a more refined one, for sure.
F5 was a launch game with a very bad sdk and most probably they had not enough time to polish it like they wanted, still achieved a 1080p60 racer on a console where many throughout last year doubted it wouldn't be able to achieve 1080p, leave alone with 60fps.
My guess is we will see quite a huge improvement in terms of lighting and content. Perhaps even day and night cycles and weather.
It won't be better looking than driveclub for obvious reasons, though.