There has to be a reason to keep the city safe, so Bungie really does need to look at visceral institutes that appeal to players. A cantina that appeals to Star War fans. A stone ring in the center of walk way leading to a library that might appeal to Halo fans. A night club that reminds GTA and Mass Effect fans of their haunts. Colorful and lively NPCs moving around in succession like out of an Assassins Creed city, simple gut reactions to the city will determine its worth to the players are important to the games longevity. If its a night club that makes you like the city, ...that's a valid reason for not just the character of the player but the player themselves that has a something to defend the walls for.