In the current economic beta, everyone is using their essence to spread their civilization as fast as possible, something that will probably be a very viable strategy in the full version as well. You scout new areas, found a town in a strategic location, spread your influence through growing your town, and make smaller towns at the border of your newly expanded territory, thus spreading even further. That results in a huge population, a lot of gold, a very high research rate and access to many diverse resources. It also helps your sovreign level up, as it's inevitable that an expansive civilization will have more conflicts than a small one, not to mention that it'd have a much better ability to support such conflicts.
Now, in order to balance that vs staying small and focusing on magic, you could make magical research essence based. Magical development wont expend your essence but it's speed will be determined by your sovreigns essence value.
The result would be that if a sovreign expended his early essence on land grab tactics, his initial magical research would suffer for it. A sovreign who used his essence sparingly would be able to gain spells and advantages to counter the land grabber's human soldiers. Summoned units, enchantments, and the like.
As for spellcasting, a sovreign should only ever really expend essence when imbuing the land or casting fairly powerful spells, with permanent effects. Instead, a sovreigns essence could determine the size of his "Mana" bar, so to speak. A sovreign with 20 essence would have a life mana bar of 20, and those 20 points could be used, then regenerated over several turns. I imagine the regeneration rate could be determined by the maximum essence of a sovreign, maybe alongside a stat on the sovreign.
Anyway, the result would be that a sovreign who only imbued a limited amount of his essence would have earlier access to powerful spells and be able to afford the upkeep of more summoned units and aura\area spells. If you took it one step further, and allowed for spells that could summon "Garrison" units of shards, units that would harvest shards the same way a city could and act as protectors, you could make it so that it would be a viable strategy to never found a city at all, or to stay at one magically optimized citadel.
On shards, Imagine that for every type of shard, you gained a seperate "mana" bar. These "mana" bars should regenerate based on your shard harvesting efficiency (something I think should belong to the magical development screen) and again your sovreigns maximum essence\10*regeneration stat. The shards "Mana" bar's maximum value should also be limited by how much essence your sovreign has. This would make shards less valuable to someone who spread their civilization quickly, and more valuable to someone who relies on a few cities, or a single citadel.
Thus, with essence determining your "Mana" regeneration rate, your magical development speed and through a maximum mana value - the potency of your spells.. essence management suddenly becomes something even empire rushers need consider. Anyway, that's my thoughts on how magic\civilization balance can be achieved.
