Instance challenge is a particular hot-button topic right now, I think, because generally in FFXIV content is either mind-numbingly easy or ball-bustingly hard. There's usually no middle ground. I agree that adding a WoW-style Heroic or Mythic difficulty to the table with each dungeon introduced would be a good idea, instead of the current method of re-introducing an older dungeon as a 'Hard Mode' in future patches (which isn't always a harder variant of it's older counterpart... glaring at you, Hullbreaker HM).

It'd be easy enough to sit down, design a dungeon, then create a few variant mechanics and mobs with greater stats to plop into the dungeon as a Heroic mode that gets introduced alongside new dungeons. That way veteran players who clear the normal content quickly have something else to do besides mindlessly farming tomes or bashing their heads against Savage content.