elemental_mana_pool

The basic idea is that e.g. fire spells require fire element energy, represented by a regenerating pool of "fire mana/energy". Each spell class draws from its own elemental pool(e.g. poison skills draw from "venom pool", cold spells from "ice energy",etc)
Specific utility skills may transfer mana between elemental pools and perks may add bonuses to specific pools(element affinity).