I hope that time they have the great idea of not having the tactile screen (i think they should keep the ds form factor cause it's really important in japan) proportionally as big as the top screen. You don't need the bottom screen to be that big on the xl, AND it would give you a bigger place for buttons and sticks, and a way better ergonomics.
That's basically the free form display Nintendo is using from Sharp, which also has ultra-thin bezels. Imagine a screen that stretches from side to side of the device, with cut-outs over where the sticks and buttons usually are. So it's wider than 16:9 (or 5:3 on 3DS), but you've got all this space around the buttons that's actually touch sensitive and a screen.
This makes for a flexible set-up where the best developers can bring the buttons and D-Pad to life via an interface, tutorial or gameplay elements that point to them and engage with them in real-time, or they could place touch sensitive buttons or actions that easily reachable from your thumbs (e.g. you could comfortable control an in-game camera via swipes). Or they could place HUD elements away from the main viewing area where a traditional screen lies.
And for lazy/resource limited/time limited developers, they can just extend the field of view for their game and claim it improves immersion. It's really flexible.
If there's one legacy the NX handheld will leave it'll be in bringing buttons and D-Pads to life, rather than them being these non-interactive thing off the screen. Should also be good at making these button controls feel natural to a new generation of players in Japan who might start playing on a touch-only smartphone or tablet.