The real “killer” MR app, IMHO, is an app that allows multiple people playing and seeing things together.
Imagine 6 people playing a shooter game in a basketball court. They are holding controllers that can be light saber, flash grenade, rocket launchers…etc. None of those weapon exists, but they feel real in the players’ hands. You will see the rocket shot at you from other players. You will see the swinging of light saber in your own hand, and you will truly enjoy this game, because MR will add so much fun in this game.
Imagine 1000s of people in an arena watch a 3D presentation in the center. Everyone see the same awesome things at the same time. It’s way more exciting than the regular concert.
We don’t have that kind of apps yet, but if you want to talk about a “killer” app, that should be it. MR is ill-suited for solo gamers. Its true strength lies in the “togetherness”.