Well, I'll just say stuff that isn't on the rumours (so no "hybrid" comments since it's a given) and isn't already on Wii U/3DS so we can assume it's also a given:
- Trophies
- Digital content linked to the account and not the hardware
- Screenshots, game video recording if possible would be a plus
I would also like it to be cheap,region free, free online, etc, but I don't consider these as "system features", they are more like business decisions.