Here's an interesting thing about the Switch (and Wii U tablet pad) screen size. If you position the screen in vertical mode the screen width is the exact same width the 3DS top widescreen is in its normal (non XL) models. Right down the exact milimeter I think.
(The above setup but with Joycons on the right/left side of the screen.)
What that means is held vertically the Switch could accomodate 3DS Virtual games fairly easily and DS even more easily.
There would have to probably be a new plastic "cradle" made so the Joycons could be connected to the vertical sides. The one thing about this though is the big vent would likely be partially covered by one of the Joycons. But since I don't think DS/3DS emulation would be so taxing on the Switch perhaps it doesn't need to vent any heat.
Nintendo also would need to make a Joycon with a d-pad at some point, but I think that's coming and maybe a stylus, though maybe some games might not work because of the differences of capactive and resistive screens (though I think this could be tweaked to work).
