Multiple revisions of the Switch are replacing the 3DS.
Already, quite a few people choose the current Nintendo Switch over their own 3DS to take on the go.
Here are some ideas:
I would love either the Switch Mini or the Switch SP from that mock-up, both are pocket sized. Between the two, SP seems like the more likely course for Nintendo to take, the clamshell design helps to protect the screen from scratches while it's in your pocket, and it would allow them to keep the detachable joycons and tv dock, while the Mini design wouldn't. Though if I was them, I'd make the screen on the SP a bit smaller, and build some recessed areas into the bezel, so they can have proper analog sticks instead of circle pads.