Actually, the Wii U can run next gen engines. CryEngine 3 was/is up and running on it, Unreal 4 can run on it. I'm guessing that FB3 can be made to run on it, with proper coding.
If capable developers like Treyarch were able to get the "HD" Call of Duty games to the Wii, there is no reason for another engine not to be able to make it to the Wii U (especially if they are running mobile versions), outside of a developer not taking the time, or knowing how, to do it.
We already have games like Trine 2 and Need for Speed: Most Wanted showing the Wii U is capable to deliver as good as, and better than, performance of the PS3 and Xbox 360 (which, yes, it should).
But, I'm not sure why people think the Wii U can't run "next gen" engines, when CryEngine 3 is running on it, and Unreal 4 was confirmed that it could be done on the Wii U, just that Epic (who didn't develop a Wii game) wouldn't be making a game for it using it. Perhaps developers should stop making excuses, and start providing examples. I believe that the first party titles will probably end up looking the best (like Metroid Prime 3 and Super Mario Galaxy for the Wii) though, while some third parties continue to act like they have no clue how to properly program/port.