I'm not sure this is the right place to talk about quests, I didn't find existing thread about it. In case it would already exist and I'd have missed it, I apologise. 

So far I've heard about RPG like quests, for the heroes. That's cool, but could there also be quests at the ruler's level?

One thing I thought was interesting in Age of Wonders was the god system. As soon as you had built a temple, you gaines bonuses but the corresponding god could give you missions. Rewards depended on the gods (artefacts, gold, spells, etc.), and they could get angry in case of failure. These quests ranged from "eradicate heretics" (kills stack of units), conquer cities, make peace, etc.

In civilizationIV's addon "Beyond the sword" appeared the "events" system. Some "events" were quests (build cities on every continent, conquer a specific ressource, build X buildings, etc.) or random events with the opportunity to chose how to handle them (a slaves revolt, do you restore order in blood? do you wait and see? do you abolish slavery? etc.). I think it was very cool and it can be easily implemented with the XML/Pyhton system about to be implemented in Elemental.

As sure as the RPG quests do enforce the immersion in the RPG aspect of the game, I think these quests can help feel like a real ruler, with decisions to make about how to rule ^^