I bet it'll have to do with a Vex mind attaining near godlike capabilities. Mercury will have a patrol, it'll all be based around preventing said Vex mind from gaining these powers at the center of Mercury beneath its machinated surface. Brotherhood of Osiris will be present to assist us, maybe we might even see friendly combat AI start to inhabit planets and get into fights with the enemies. Maybe even quests in the field from small outposts and such since the City will have more resources now that we're stronger. The Vex will be the main enemy, perhaps the Cabal will gain a stronger presence on Mars and most of the Vex in system have gone to Mercury to protect their mind.