It'd be really cool if these were "last stand" story missions. That is to say, missions near the end of the story, when the darkness is at its strongest. The vangaurd has dispatched all Guardians to go on an important mission. These missions would be innately Heroic, and have different sized fire teams based on class. Loot would be distributed as evenly as possible so as to not make one mission more favorable. Titans, like you said, are fighting endless waves of enemies. They are stationed at the wall outside of the city, where the Fallen and Hive have joined under the Darkness to take the city. Hive-infested walker tanks and massive ogres make appearances. They appear to be led by an Archon and a Knight. The Titans, in a match-made fire team of 6, are the last stand, the only thing keeping the enemy from taking the city. Joining them on the field are AI controlled Shaxx (Striker), Saladin (Crusader), and Zavala (Defender), making for a group of 9 Titans in combat. Warlocks have been dispatched to recover the final shards of the traveler from the Vex, traveling back and forth through time, as well as between Venus and the machine world of Mercury to do so. The final shard of the Traveler needed to repair it is on Mercury, guarded by a Vex Drakon, a serpentine beast that moves quickly despite its size. Its thick hide must be stripped away before any real damage can be done to it, but it must also be prevented from reversing time on itself to heal, via destruction of focus crystals stationed around the chamber. They are sent in a match-made group of 3, joined by the Stranger. The Hunter has been sent on a black ops solo rescue mission. The Speaker has been kidnapped, and believed to be somewhere in the Reef. The Hunter is to get in and out quietly to search for the Speaker. Upon arrival, it is found that the Queen herself was also captured, and that a Cabal prince has taken her throne, allied with the House of Wolves. The Crow, despite his distaste for you, joins your side as you assault the Cabal and Fallen to save your leaders. If successful, the Crow guarantees the Queen's aid in the campaign against the Darkness. Edit: to add to this, each faction (DO, FWC, NM) will have a smaller wave defense mission, with the mission focused on the goals of the faction. Rewards are mats, rep, and glimmer, as well as engram's from enemies. FWC just wants war and needs more resources to sustain it. DO needs resources to build their escape fleet. NM needs resources to establish a new state.