wii-u is btw a game tablet.
Maybe if you have a very loose definition of "game tablet." The Wii-U still requires a console box and TV. The screen on the tablet controller is used a second screen, not as a replacement for the TV. The tablet controller was also built with buttons and controls, the way a handheld would be. If anything, the Wii-U is more like a merging of a handheld (like the DS) and a console.
Tablet gaming devices like the iPhone or iPad would have to incorporate those kind of handheld features (joystick, buttons, d-pad, etc.) in order to be considered a serious gaming device. That would kind of be moving backward, since the whole reason those devices were developed was to REMOVE the button functionality of previous devices (phones, computers) and make use of touch navigation.
It might make sense for Nintendo to move to tablet gaming in the near future. Because Nintendo actually focuses on casual games without caring much about featuring superior graphics, it would make sense to try containing that experience into a tablet device. Nintendo's current selection of games is mostly in the same realm of games that people already play on tablets (that is, games that can be picked up for 5 minutes at a time).
It might also make sense for
handheld gaming platforms to move in the direction of tablets. Nintendo has the 3DS, Sony has the Vita. BOTH feature touch controls in some way. Microsoft currently has no dedicated gaming handheld. It would make definitely make sense for Microsoft to develop a handheld
alongside a regular console if they chose to do it, and it's very likely that if they did, it would incorporate touch controls in some way.
But it does not make sense for Microsoft to attempt to replace a console with a handheld tablet device. The DS in no way replaced the Wii. The PS Vita in no way replaces the existence of the PS3. Even current tablet devices are not a replacement for serious PC gaming. Using a console and using a handheld gaming device are two completely different experiences for a gamer, and should be treated as such.
So if Microsoft does want to start developing a handheld gaming device, then that's
definitely a possibility, but it would be silly to expect a handheld device could
replace the console experience at any time in the near future.