They're not adding a second screen to any switch.
3DS and DS emulation is probably possible, but Nintendo would have to find work arounds instead. Perhaps having them show up side by side, or pressing a button to switch between screens.
That being said, I wonder when/if they'll introduce wii/GCN games anytime soon. Perhaps will be one of the features showcased with the pro. The shield and NVIDIA's ports show the hardware is pretty capable of handling wii/GCN games in HD, so it'll be interesting if they can do even more with those. 1080p GCN games on the handheld would look pretty crisp
Oh man, that would be a great selling point for the Switch Pro that i completely forgot about. I thought the improved BC was a great way to incentivize upgrading to the Xbox One X as well.