Okay, feel free to tell me this is silly.  Spontaneous heroes come from idle training structures.

I got the idea from Colonization.  Idle structures train the populace, increasing output.  Idle training structures train the populace for faster unit production.  OR they put points into a pool for heroes.  The type of hero produced will be random, based on unused weapons and armor in the town stockpile.  Where the idea isn't silly: you make a choice between constant production of military units, or the slow, random production of heroes.

It also fits nicely into my concept for non-combat heroes.  All idle structures add to the pool, and if a hero arises, she may not necessarily bear a blade.  Maybe she aids crop production, or milling, or helps to raise horses/bears/skysharks. 

AND I've refined my idea for Council/Mayors.  We put in some civics technologies, thus allowing for positions in each town, and open the seats of the Ruler's Court with other technologies.  This allows us some leeway in balancing the Court - only players who WANT a Minister of War will research toward it (possibly picking up other war techs).  I'm also working out how to balance 'flexible' seats.  For example, maybe I want a Minister of Bears so that my entire empire is better at producing well-trained Bear Cavalry.

AND I had an idea on random hero advancements: they work similar to non-specific tech advancement from Alpha Centauri.  You can specify where you encourage your heroes to train, but if they REALLY want to ride a skyshark instead of a pegasus or bear, you don't have ABSOLUTE control over it.  OTOH, I can see where this should be an OPTION; I don't want to have a force of six heroes, not one of whom can channel mana into special effects.