Oh now here's an idea that might be worth pointing out. The successor to the 3DS could serve as a gamepad for Wii U backwards compatibility. I mean, the 3DS can already function similar to a gamepad with Smash. So if the successor doesn't have the gamepad bundled in, they could sell it separately (bundling it later alongside a traditional controller once production costs fall and if it is popular enough) or you could use the handheld as one. Also if the streaming tech is in it, then the handheld could stream games to *and* from the console, so you could put handheld games up on the screen or console ones on the handheld.
Also, the gamepad imo is not a gimmick so much, it just hasn't beeen used well. It has the potential to allow for games that couldn't be done previously without additional peripherals. For example, a number of advanced mech games require a lot of buttons, much more than the 14 standard inputs on normal controllers (2 triggers, 2 bumpers, 4 face buttons, 4 directions on the d-pad, and 2 clickable analogue sticks). The gamepad, however, can easily fit 20 new inputs on the screen that wouldn't be too dificult to reach, giving the game 34 inputs. That's just one example.