In the Crucible, there is a game mode called Trials of Osiris, which is arguably the most competitive playlist there is, even more so than the Iron Banner. To compete in the Trials, you will need to have reached level 20, and pay an entry fee of a mote of light. After three losses, you are kicked from the playlist, and will have to pay with another mote of light to re-enter. This game mode is the only one in which you can play the Burning Shrine multiplayer map on Mercury. Still no info on the Seven Seraphs, however.