Semi-permanent dynamic events.
That already happens in some places as it is. Certain outposts will get taken over and the waypoints will remain contested / merchants disabled until someone goes there and clears the place out. Or friendly NPCs will take over an enemy outpost and as long as players defend it now and then it’ll remain a friendly outpost (happens a lot with Seraph and Centaurs).
I don’t think that should happen with major hub cities though. It would suck to be trying to do your story mission only to find out “oh bugger the city has been taken over so you can’t do anything until a bunch of people get together and kill a hundred enemies”. Or to be sitting there trying to craft items and suddenly the city is being assaulted and you get swarmed and killed. Cities should be safe zones, IMO.
And Lion’s Arch would NEVER get taken over. There are way too many people there for that to ever happen.