You know, I totally disagree with the statement that GT must go online-focus to survive. Looking at the market for pure online versus single player (even sims like AC quote that only 20% of the user base races online) I'd argue that GT must retain a broad and relevant single player mode to generate sales.
If you look at sales charts like vgchartz (which only partially covers sales) then neither major franchise is holding up sales, but GT is still miles ahead of the competition, despite GT5/6 seeming "stale".
http://www.vgchartz.com/gamedb/?name=forza+motorsport
Forza 4 global sales 4.4 million (Xbox 360)
Forza 5 global sales 2.2 million (Xbox One)
Forza 6 global sales 1.7 million (Xbox One)
http://www.vgchartz.com/gamedb/?name=gran+turismo
GT5 global sales 10.7 million (PS3)
GT6 global sales 3.6 million (PS3)
(yes, Sony listed GT6 sales above 5 million, but I'm compared vgchartz like-for-like)
GT6 managed to sell effectively the same as Forza 5 and 6 combined.
I'd argue that GT7 with significant graphical improvements (4k+HDR on PS4 Pro) and PSVR support would sell like hotcakes to everyone wanting to show off their kit. And the most important feature would be car collecting, simple career mode, and lots of awesome replays and photo mode capabilities.