Roleplaying and creativity is pretty high on my list. For example I'm working on a Quest Design system where you can hook together anything in the world to create a quest.

A simple example:

You create a Wizard NPC with an input of (4) (Boars tusks) (which you link to the Boar's monster area). Once completed it unlocks the (Gate) next to him, allowing the player to access the tower full of loot. The inputs and outputs would be completely flexible and could lead to some pretty funny situations. Careful balancing would be needed so players aren't walking halfway across the map just for a shoddy Wooden sword reward.