Arkaign on 12 September 2015
I think it looks way better than Forza 5, which was a bitter disappointment to me personally.
That said, I doubt it will outsell Forza 5, as there are a surprising number of people who will just skip a release in an iterative series like Forza, GT, Madden, etc. But more importantly, it's releasing right in a crapstorm of content this fall that even while not directly competing on the race-sim field, more than likely will have most gamers priorities in purchasing.
This fall's lineup blows away 2013 and 2014 by a long shot for console imho.
My guess for Forza 5 total lifetime sales : 3M (if they mark it $19.99 value or whatever and keep it on shelves/online)
My guess for Forza 6 total lifetime sales : 2M
