I haven't finished the game yet, so maybe some of this happens, but...

Since I started playing, I've wished we could explore more of the city. Today, I was camping near a spawn point with some other players, just killing waves as they arrived, and I realized how fun it would be if there were a sort of trench warfare vibe in an area, with just waves of enemies arriving from a single direction.

Maybe the Cabal could attack the City at some point. Part of the City near the wall could be playable, with enemies on the other side, and maybe occasional events where they rush the wall, and you have to hold them back. Might be difficult to fake, since areas don't really change dynamically, but I can already think of a few ways they could do it...