A dozen cars or a pair of new tracks every so often - or dare I say it, a Classic Track Pack of the missing locations from the previous GTs, that won't hinder work on GT6 at all. And GT6 is undoubtedly going to be much different than GT5 with a bunch of DLC. How much is GT5 like GT4? And yes, I know GT4 is on the PS2, but if it was up to Prologue levels, would it be good enough to pass up a Gran Turismo with Ferrari and Lamborghini, Spa and the expanded Nurburgring?
As for GT6 on PS3, it's possible. If the team can work some magical optimizations to squeeze even more power out of the Cell architecture - without melting it, then a lot of issues can be fixed. Say, making the 3D engine an optional install, because I'm sure the 3D applications are part of the reason that certain aspects of Prologue look better than GT5. If shaders can be better optimized, lighting - especially at night, particle effects not interfering with the rendering of objects, and so on, then GT6 could be a much prettier game. Work on integrating the online system with a fleet of servers around the world to make online nearly seamless with the offline splendor.
However, a PS4 would definitely be a quantum leap in technology over the PS3. With a super processor structure, a fast architecture with abundant ram, and a cutting edge graphics chipset, Polyphony could do some pretty stunning things. Perhaps Digital Molecular Matter technology could work on the fly to implement real time deformation and damage near the level of a current supercomputing simulation. Body crumpling, paint scarring and component failure wouldn't be pre-baked and loaded when certain parameters are met, but calculated in real time based on realistic collision dynamics.
Physics could equal or perhaps even surpass what's currently running on PC sims. Track surface undulations could be accurate to within a few milimeters, and directly impact tire behavior and grip. Car behavior could be as close to real life as it gets. A Livery Editor could be off the charts powerful, allowing you to create any livery from history or your imagination, and performance mods and bodykits allow you to Race Mod every car in the game.
A.I. could be near the level of human intelligence, only slightly different from racing against people online, so you wouldn't have to go online and face a random list of servers with questionable players or only have a few friends handy. The field would be full and competitive.
Weather could be modeled on real world data from the actual locations, and rendered more lifelike. Showers could become downpours, and the moisture on the track surface could build up and puddle realistically in real time. When rain stops, evaporation rates could be different based on whether the sun shines on the track or is obscured by clouds, and on the time of year and temperature. Rain could become snow if you wanted. There could be forecasts for upcoming races in a season-based calendar.
There could be 24 car race fields or more, allowing for challenging multi-classed races. There could be an entire simulator mode with racing from various leagues, along with a calendar which tracks your progress and points standings. A massive data base would allow you to see what each bot has done, his race results and standings, as well as keep track of what you did with each one of your cars. Event Building tools could allow you to make up your own championships for offline fun, or online events and league racing with private or open clubs. Online Club Builder tools would allow you to form clubs, clans and leagues. Storefronts and an online Auction would allow you to sell cars, liveries, decals, tunes, whatever.
A Course Maker could be even more powerful than the one in ModNation Racers, allowing you to create any semblance of a real world track or fantasy track of your choosing, and be quite elaborate if you devote the time to it. And Photo Mode would be stunning, along with the possibility of a Movie Maker to document your races and share the videos online or over YouTube or other services. And who knows how far the possibilities of a truly powerful next gen PS4 could go?
This is why I expect GT5 to be expanded for two or three years, and GT6 to be on the next system.