Polyphony Digital boss and Gran Turismo series creator Kazunori Yamauchi tells Kotaku that despite throwing everything from karts to damage at Gran Turismo 5, still he wanted more. For that stuff, wait for GT6, due sometime in 2019.
People seem to forget that GT5's long gestation period was largely due to building the entire engine from the ground up. New physics, new AI, weather, etc. This, in addition to it's wealth of content and being on an entirely new console for the first time. With that finalized, all that's left for a sequel is tweaks, polishing, and more content.
It's a lot easier to add to something that's there than it is to make it from scratch. Maybe the physics will be tweaked, a few more car companies will be added, damage will be more polished, new circuits, events, etc. They basically just get to pile all the goodies on top (although GT5 has plenty of goodies in it's own right).
And even at worst, if the game takes another 5 years, that puts it within this console generation still, but I'd expect to see a sequel in 2-3 years max now that the bulk of the work is done.