Nintendo is in a complex position. Right now if I was in charge I'd be frantically putting together an anti Wii U that could be released within two years. If the Wii U doesn't pick up they need an alternative as fast as possible. If things start to make a turn for the better, that project can be put on hold while a more thought out and original console is designed. I'd definitely look into building the system around a gimmick, but that wouldn't be a priority. The 3DS's gimmick was a failure. Now the system is selling just because of the great games. The Wii U gimmick is failing, and all the great games don't need it. Motion controls and to a much lesser extent the DS multi screens were one off successes that are unlikely to be repeated. I'd say it's better to focus on other innovations like online features than keep trying to re-find the Wiimote magic.
But, having an out of the way gimmick certainly doesn't hurt. The PS4 has the light-bar, touch-pad, and speaker while the XBONE has haptic triggers. Except for the DS4 light-bar not turning off, no one has any problems with these features. Sticking an equivalent gimmick or two in the next system would be ok. It just needs to be secondary, not the main attraction. A gimmick I'd personally invest is force feedback. Not the rumble on the XBONE triggers, but actually a force pushing the buttons back at you. Basically variable resistance. It would be pretty cool I think as a next gen version of rumble. Every game genre could support it too.
Interconnectivity between the new system and the 3DS would also be important. I find the concept of a portable/home console hybrid ridiculous, but incorporating aspects of that idea makes sense. I'd stick very cheap processors in every controller. In home use this allows some gimmicky features like assigning a user to a controller or turning on the controller when the console turns on. Combined with traveling however it would act more like a memory card. Take it to your friend's house and your profile follows you without the need of logging in. It can also have streetpass functionality with 3DSs. Sorry but you can't play just the controller. For that you'd need an LCD add on or a smartphone. Game would just be streamed to you. This specific feature really wouldn't be big, but it would be silly not to support it. Local multiplayer with select 3DS games would also become possible. Again not a feature that'd sell a lot of consoles, but it'd be silly not to support it.
Or something else.
