So the purpose of this suggestion is twofold; first and most importantly it is to make strong economy builds a viable option, rather than simply making someone a plunder farm.
The second is to add further customization to a province/playstyle and add a further layer of depth for min/maxers to tinker with.

The idea is kind of a cross-over between Dragon's and Ritual's, and would be unique on a province to province basis rather than shared Kingdom-wide.

Wyvern's! These are dragonesque creatures far more tamable a trainable than their larger Dragon cousins.

A player can opt to invest a selectable portion of their net daily income (up to perhaps 75%?) in the raising and training of a Wyvern to benefit their province.

The more gold the player earns/invests, the more powerful the effect of their pet Wyvern. Similarly to how you can overcast a ritual with runes, you can increase the effectiveness/loyalty of your pet Wyvern by investing more gold than necessary. If a player decides to drop or increase the rate of funds, the Wyvern's effect will increase or diminish over time, similarly to how building efficiency changes relative to your number of workers.

The Wyvern's effects might be similar to Dragons, except beneficial to yourself, instead of a hinderence to your enemy.

For example:

Amethyst Wyvern +x% science effectiveness
Emerald Wvvern -x% military losses in combat, +x% attack gains
Ruby Wyvern Increases military effectiveness by x%
Sapphire Wyvern +x% higher magic and thievery effectiveness
Topaz Wyvern Increases building efficiency by x%

A player can raise only 1 Wyvern at a time but can swap at any time by releasing their current one, paying a small upfront gold fee to raise a new one, and then continue to invest daily net income to train it and increase it's effectiveness/loyalty.