We are seriously overdue something that will allow us to adjust the internal game clock without having to exit the game to adjust the clock on our handheld. It's not a hugely revolutionary feature and, if day and night are important factors in these games (as they may very well be given the names) it'd make sense for whatever the new device is to have a function that would allow us to change the time of day.
Sure, it wouldn't make any sense in game, but it would provide a potential whole new depth for in-game puzzles in caves and the like, and would also let us adjust the time of day to catch specific Pokemon, etc. Constantly screwing with the clock in the fourth gen was a real nuisance. If these games have day/night - and I really hope they do - we need a feature like this.
It's unlikely, but video chat on top of voice chat would be cool, too. I'm guessing these things will be connected to the internet at all times the same way XY were, so having video chat between friends - and only friends - would be a nifty little feature.