I honestly doubt that the successor of the 360 and PS3 will be "much more powerful" than the WiiU. One reason is because there's no way to gauge what "much more powerful" means unless you put it into technical terms. When I think "much more powerful," I think 6-8 times more powerful. I do not think it will be 6-8 times more powerful than the Wii U when the Wii U is already as good as confirmed to be more powerful than the PS3 and 360 as it is. Not only this, but there is a user here who constantly actually makes some good sense when talking about this issue.
1) Making a system more powerful requires more investment on both Microsoft/Sony and the developers for the system, which means much more money spent. This isn't a good idea considering their financial loss history and how it will just keep on going unless they do something about it.
2) Making a system more powerful not only costs more money for the companies, but will drive the costs up for the system. If the Wii U ends up being something like $349.99, then the "much more powerful" system will end up being "much more expensive" as Nintendo has started going with the trend of making their systems cheaper for what you get with your money.
3) When you use more power in a system, the system exhausts more heat. Withstanding more heat means providing better cooling and higher quality materials to keep the system from damaging. In some cases, the power would be nearly impossible to cool down to a proper temperature unless a really expensive cooling system were put into it. This would mean bulking up the system, driving up costs, and more potential for malfunctions resulting in lower ratings (and lower ratings means less people willing to go out and buy one)
4) The graphics now are already nearing a cap to where we can't tell the difference, and the only places where we can really improve a lot on are things like AI.