First is an online browser game. It is single server, so everyone who joins starts at the same place. It will include puzzles that must be solved without communication (no chat or voice). Most puzzles will include sacrifices where some must be left behind (for example a pressure plate that opens a door only while someone is standing on it). The highest room that can be reached is therefore determined by the number of people and cooperativeness of said persons.
Second idea is an app that will track your location. You can then choose to “drop” a message at your current location. When users get close enough to where you wrote the message, they can choose to display the message on their screen. They can like or dislike the message. Messages will be more prominent the better their like/dislike ratio is. It can even be implemented using AR to spice it up a little.