My crazy idea: how about a second screen as accessory you stick in the USB-C-port (so at the bottom of the Switch)? No own battery, it would get its power through USB.
There could be a very light and slim model (only the touchscreen) for digital 3DS and DS games and a thicker model (still light enough to feel comfortable) with a slot for DS/3DS cards.
That idea would solve the Dual Screen problem with playing the games on the Switch. Something like this I can see Nintendo possibly looking into if they ever decide to merge there markets together and include this feature on the Switch. Its simple, they don't have to butcher the games to work on it and it looks modern.
These GameBoy BC features worked incredibly well for Nintendo in the past and this idea has no real negative feedback if they decide to go for it.