Soundwave on 23 January 2017
The Joycon grip with the "puppy ears" removed could likely be turned into a clamshell design with a few tweaks. You'd likely have to flatten the joysticks to be a bit more like 3DS sticks, but otherwise just slap a screen with a battery underlay onto this and you have a relatively small portable machine that doesn't break compatibility with any Joycons.
Likely they'd have to wait for a die shrink, but it's probably doable at 14nm or 10nm.
