Couldn't find the list, but here's some Ideas
I could probably think of more If I had to, but share your thoughts?
Pedometer-Strengthen Bonds between Pokemon
-Hatch Eggs
-Pokewalker-esque areas around the region?
Camera-Custom BOX Backgrounds
-Front Camera could be used for the Trainer Card
-Trainer Card's Image could then be used for the Hall of fame
Play Coins (Coins saved up from walking every 100 steps)
-Buy special/rare items in the shop
-'Coin Launcher' Mini game to get Various in-game prizes (Gambling Alternative)
Streetpass-Pass by others and get their Party Data and Battle Stats
-These Trainers would show up in a 'Trainer Park' for battling; once a day unless you pass by them again and get updated data.
-Shows up in game (This is where customization would be great for the games.)
-Join Avenue 2.0?
Spotpass-Spotpass Trades
-Trade with out the other person being online.
-Spotpass Battles
-They would likely show up in the same area as the Streetpass Trainers
-Gift Pokemon (No longer needing to catch a Bidoof or a Patrat for the Pokemon)
-Improved Mail System
-Send mail without the restriction of words and body styles
-Each trainer would receive mail at their home or maybe the Pokecenter
-Attach Items to letters (Fixing the fact that you would need to trade a Pokemon to gift items)
-Automatic Mystery Gift Downloads
Motion Sensor/Gyroscope-Adding a level of complexity to Minigames in similar fashion to the Pokeathlon/Contest/Musical
-Something Similar to making Poffins?
Miiverse-Sharing Achievements (Hall Of Fame/Pokedex/Others)
-All the other things Miiverse is intended for...