This is a mod I'd really like to see, I'm a novice at best coding this game (I'm just getting started) so I have no clue how you would go about doing that.

This is the type of mod that would go great with single use spells, like certain summons, where you could cast the spell to get the effect but wouldn't be able to cast it again until the effect ended, or casting it again would over-ride the previous effect.

A single use spell (of the cast only once variety) that would be truly awesome would be to summon a permanent map resource that gives 1 of every elemental crystal once claimed, at the permanent cost of around 15 essence.

Come to think of it, the ability to permanently subtract essence makes it possible to introduce almost any map resource via spell without unbalancing the game. Just make essence cost suitable for the resource and you're in business.