The most powerful console of the last generation was the Xbox 1 with its GeForce 3. The Playstation 3 has a GeForce 7. So it would seem that to get the same gap in performance we would need to go up 4 generations.
8800
GTX 200
GTX 400
GTX 500
However, let's also consider the Xbox had a single core 733MHz Coppermine PIII/Celeron. Even the CPU in the 360 is vastly more powerful, let a lone the 6 core beast in the PS3.
It is a certainty that the CPU increase from the Xbox to PS3 generation far larger than PS3 to PS4. Therefore, in order to get a similar overall performance increase, we will need at least another GPU increase.
So at the absolute minimum we'd need to see the Radeon 7000 or GTX 600 series in the PS4/Xbox 3. Considering the relatively small gap from Radeon 5000>6000 and GTX 400>500, we would more realistically need to see a Radeon 8000 or GTX 700 series to get the same relative performance increase in the next generation.
If the new consoles come in 2014, considering they need some time to work with the architecture, I'm guessing it'll be a Radeon 7000 or GTX 600 series, though hopefully we do see a step higher.
So we're not quite at diminishing returns, but without a significantly better CPU they are gonna need one hell of a GPU in the next gen to see a similar increase.
Btw, more on topic with the OP, the WiiU has a Radeon 4000. So if it is only 3 GPU cycles behind the Xbox 3 or whatever, then I would imagine it would be closer in performance than the Wii was. Especially considering the Wii had a single core CPU and the new one will have a 3-4 Core PPC. So I do not think the WiiU will be in as serious trouble as last time.
Who knows at this point, but going from PS4 to WiiU may be relatively as simple as turning a PC game down from Very High, to High or at worst Medium. I don't think we're gonna see the same physics, texture, and polygon deficiencies as last time. Every game should be as equally as playable and simply have slightly less impressive graphics.