empty osprey
Not sure if has been already mentioned (in that case sorry for the post).
Possibility to create guilds, with related achievement, cosmetic, embles, and mount to unlock (by completing tot number of dg or killing certain number of boss/mobs etc).
Expanding the stronghold and have an housing system (limited to the single player or guild house). You can have housing stuff dropping from dungeon or purchasable with gold. Which can be traded with other or purchased by other player with an in game auction/market system.
The guild not only will improve the social aspect but add another layer of things to achieve and get while grinding dungeons.
Guilds could also resolve queue and grouping issue, which seems to be the case especially in the Eternal tier.
Housing will give more things to do while outside the dungeon loop