One thing I would absolutely love would be a proper prologue demo of sorts, similar to what Bravely Second had. How about playing as a different character - maybe the Professor, or a member of the Elite Four, even! - and experiencing things from their perspective?
If these games were sequels to X and Y set in a different part of Kalos, playing as Malva, or a low-level Team Flare grunt on a special mission from her, would be amazing. They could set up a really engrossing story for the main games through a demo like this, something that Pokemon has never really had in my opinion.
Anything is better than nothing at all, but if we're wishing for things I'd like something a little more substantial than ORAS' rather lackluster attempt at a demo.