You asked for more uses for the gems, Captain Jack.
I can think of several options, but the problem is that these options are either covered by existing voodoo cards (like spend 1 gem to gain 1 point on a ship, or spend 1 gem to gain 5% more profit in a specific port for 24 hours, or spend one gem to increase the population in a specific port), or it isn't in the EXISTING voodoo deck, but would better be covered by a new voodoo card (like spend 1 gem to make your next voodoo card gain the "undercover" ability, or spend 1 gem to see the danger on an enemy fleet, or spend one gem to be allowed to have 6 ships in a specific fleet for 24 hours).
You could, of course, demand gems to level up the top tier ships (SoL and MoW), but that would make them too expensive, I think. Or buildings could cost gems, but that won't increase the demand much, as a building level is built only once, and that's that.
Basically, I think gems fits better in with new features. The witches hut is one. If other new features are implemented, then they, too, might require gems. For instance, if players can start producing goods (in addition to food), then perhaps a "Craft Master" is required to keep it running, and he requires one gem pr week as payment - or a number of gems, depening on your production? What use they may have in a new feature is hard to tell, of course, since we don't know the details so such new features.