The wall doesn't defend itself. Imagine a game mode where you become called upon to go defend the last city's walls. We could get more lore on the wall itself. We'd get closer to the city. And we'd get the hoard/survival mode a lot of us want. The Vanguard would be called upon in times of crisis ex:enemies massing at the walls. So the game mode would be for those of at least vanguard rank 1 (or 3 depending on what the devs do). You could also level lock it to 20. Maybe have it with a lot of people (upwards of 20+) and each group of six would have smaller tasks to perform to ensure the wall stays in Human hands. For example. Say Commander Zavalla gets reports that the Cabal are using a mining machine to attempt to break through. You and your squad have to stop it. While another squad is sent from the wall to find out how the Hive have entered the wall and must drive them back. I think this could be a very good dynamic game mode close to patrol. But with forced objectives.