@Frogboy
Since you asked for ideas on Essence, I posted my idea in the following thread (both kailen3's and my take on it are very similar)
https://forums.elementalgame.com/387478/#2681661
Basically:
1.) Essence is a global resource like food (does not accumulate like gold/materials, but acts like food does)
2.) It is gained through land withing your borders and special buildings/sites
3.) It is spent on:
a. Cities (To make the land fertile) - This will limit the amount of cities in the game
b. Powerful units
c.) Enchanted weapons/armor
4.) It is gained back when losing cities, enchanted items, powerful units (just like food is gained back when selling huts)
Mana on the other hand is determined by each Champions wisdom, and multiplied through powerful buildings (after research)
E.g. Sovereign has 15 Wisdom, so 15 mana
You build a Wizard Tower, which increases each Champions Mana to Wisdom x2
Sovereign now has 30 mana
(Essence could also be spend to turn Champions into 'mobile wizarding devices' for the Sovereign, in that case you could limit magic to the Sovereign, but he could cast THROUGH his champions - and thereby take an active role even in battles he doesnt attend - just like in MoM)