[HoneyBadger]

While we're daydreaming, the idea of adding gems to PD is a rather intriguing one. You could set it up as straightforwardly as current PD = 10 gems of a given single type give you 10x units for that Nation, with maybe 2 gem types allowed per Nation.

Mythologically speaking, there's plenty of references to draw from this. Various warding spells, guardian creatures. Gargoyles, Terra-cotta armies. All sorts of things, from all sorts of myths, worldwide. So, it should fit into the game without much disruption of flavour.

It's a little more micro, but not a devastating amount (2 easy-to-use systems, rather than one easy-to-use system), and would give you a much broader spectrum of options.

PD itself, in it's current "gold = PD" form, could be made more interesting simply by expanding the existing system of adding additional commanders and unit types, once milestones (1+, 20+) are met. Perhaps 2 new unit types and 1 new commander every 20 points past the first 20? That wouldn't be too complicated to understand, wouldn't require too much of a change in gameplay, and wouldn't require any more micro than we've already got.