Well we still haven't done a whole lot with the cabal, so a strike against their empire will likely happen And we haven't really bothered with the vex since VOG, and we know the axis minds control the majority of the vex, so who or what controls the axis minds? House of kings has yet to be tapped, they've been staying out of the fight for the most part, but we know they control the devils with no explanation of how or why Kinda tired of the hive but apparently there's a whole pantheon of hive gods to deal with beyond Oryx and Crota And finally, the largely untouched enemy "the darkness" with only 1 confirmed kill against it, the gardens black heart