For all of the people saying that work on GT7 is slowing down the work on GT6, it is possible but highly unlikely.
This is possible, but I'm not sure how likely.
See, it depends on what the team is up to. Modeling and arting will likely be the same laborious but excellent process, maybe a little less compressed for the shiny new world they're building for PS4.
But then there's this coding side. What if they're doing like I've been assuming, and are working to bring the world environs to life as realistically as possible? To the point that flags wave and foliage sways uniquely in breezes. People in crowds move much more independently, and not like zombies. A much better lighting model, one that supports a number of light sources, and shadows that don't look like poo. And a network structure that might be better than the one they hyped up for GT6. And that's just the start
What if they're going full out on a modification system as deep as the one in Forza, and a Livery Editor with it, for full on tuner and race modding of just about every car in the game? What if they're working on a damage system that's remotely like the one in Forza 4 or Dirt 3? This will also take some modeling time. And this is something they definitely need to do. That's a lot of work even for a much bigger team.
And knowing what little I do of Kaz and his team, I'm sure that GT7 is going to be the all stops out crescendo, and they're going to reach for the stars, so unless they make seriously well endowed updates for GT6, or a massive Prologue, I suspect that at least a few things will be pending updates for GT7.
So where does that leave us and GT6? A lot of this depends on us to an extent. If we have abandoned GT6, even to not buying it to start with in big enough numbers, SONY may well see support for GT6 as a waste of resources and we may see very little for it. Vagabond and Imari remarked about this possibility in a previous post, I did too. But it seems that even in this thread, enough complainers are willing to forgive and forget if they at the very least get some news on what's up. If it's that PD have dumped GT6 in favor of a PS4 Prologue, I have a feeling that all bets are off.
This is a very tricky dynamic. The fans feel betrayed because promises are late. Almost four months in, and not a sniff of DLC, and this was as good as promised. SONY needs a return on their investment, and if we turn GT6 into losing stock for them, they'll drop it. As much as fans insist it's up to them to fulfill every vocal contract with us concerning game content we pay a pretty penny for, they also aren't a charity and need to turn a profit. And PS4 needs a good racer badly. As worthy as Drive Club and Project CARS might be, and Assetto Corsa and rFactor 2 if they make it in, I can't see those being anything but niche racers selling a few million. Like it or not, Gran Turismo is THE racing game for not just Playstation, but gamers in general.
Supposedly we'll be getting something in a few weeks, as Lawndart finessed from someone at the GDC. A few weeks more, and the Ayrton Senna content is rumored to be due for release, and that's in E3 season. If all we hear is "Prologue Prologue Prologue," then we'll know it's time to begin packing bags and readying to move on to GT7 Prologue, or whatever racer will make them happy.
I heard a rumor about Forza Horizon 2 coming out this year for Xbox One by a 'insider', who said its getting that 'sexy weather' system.
I'll do just that. 👍 I'll try buying it alongside Enthusia. 
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