Shades of gray political plots that force the overall playerbase to pick a side. A civil war in which neither side is the clear good guy, with plenty of room for players to project upon them and enthusiastically support.
Plots about groups of players changing some part of the landscape forever. Building a castle, destroying one, conquering the orc tribes and ruling over a large monster army until retirement, etc.
Prophetic riddles that the playerbase muses over and tries to "figure out" what is going to happen, and how to prevent, or cause it to happen.