I think a very fun idea would be to implement some content such as marks, similar to FF12. To make it easy to implement, you could use the guildleve counter. Notorious monsters (NM's) of all different types and levels, could be rotated on the "Hunt Board" similarly to leves. I think the most interesting aspect of it should be spawn conditions for the NM. The board could give you information about an NM and tell you where it spawns, but leave it up to the player to deduce what exactly they need to do to spawn it. The NM's themselves should be interesting to fight, using unique abilities and having traits or trademarks.
I think the biggest reason most players are not having fun in Final Fantasy XIV, is because they are not challenged enough. We'd like content that involves using our brains, because otherwise the game becomes too monotonous and we lose the fire that keeps us playing.