As with most "themepark" MMOs, we're already seeing complaints about the supposed lack of end-game. There is simply no way for a developer to create unique content at the same speed that players are going through it. With the likes of housing, we're seeing Square Enix's willingness to diverge from the standard formula. These are my ideas on how to enhance replayability.
  1. Expand PvP: Add ratings/ranks for satisfying matchmaking and provide more game modes than just Team Deathmatch. Maybe we'll see some of that in 2.1 already.
  2. PvPvE arenas: I think we have enough DOTA clones by now, but the juggling act of battling mobs and enemy players while pursuing your objective is an ingenius idea.
  3. Randomized dungeons: What if you were stuck in a maze with no idea what to expect or where to go? Randomized layouts, monsters, traps and chests could keep you on edge while mitigating the "been there, done that" fatigue.