To really create the ultimative frame-deck, modularize the inputs with the new 16 inch rail system.
How about making it a giant 3DS?
Main screen just like laptop as top cover and then one input rail below for joystick modules (self advertisement) and a small keyboard or a secondary screen.
You could connect a USB 2.0 hub to one USB-C for the input rail and route the other 3 to standard expansion card slots at the back (DisplayPort, Ethernet and charging).
Their default position would interfere with hand placement.
And you need the battery for a mobile device. That would definitely go below the Mainboard. Probably the expansion card slots would go there too.