Have you considered adding Earth based spells or even borrow from the radiated environment with something like Radium?
What about adding elemental beings for each mastery? Beings the player must initially destroy in order to harvest important, and perhaps essential, Core Essences. Off the top of my head and borrowing heavily from old tropes... ICE: Frost Giant or Ice Golem. Fire: Magma Fiend or Fire Wraith. Lightning: Storm Sentient or Charged Specter. Earth beings could comprise of something like a Trent or Nature Spirit. And Radium might have something like, Unstable Construct, Decaying Nightmare or Shambling Horror.
You could also give the player infinities with certain dominated elementals in the form of boosters to magic, pertaining to said elemental, while in the presence of that elemental.
You could also summon these things, ( perhaps stronger versions of the ones you have previously dominated ) but with a high resource cost and perhaps also, a hit to players health and overall resistance to the attacks of other beings outside of the field of magic, ( and of opposing magics, more below ) of the summoned being, to simulate the summons is draining you. Think Raistlin from the Dragonlance series; capable of terrifying power, but weak as a kitten afterwords.
Each elemental discipline of magic could also have strengths and weaknesses depending on the biome and summoned being and cause extra hatred by an opposing elemental to the currently used spell discipline. So for example, you summon an Ice Golem in the snow biome and it and you are more powerful, than lets say, you summoned the same being and used the same magic in the Forrest region.
Conversely, Forrest Elementals spawn more frequently and attack with more ferocity because they consider frost a treat to their existence in their biome.
So you could do, Fire and Ice oppose, Earth and Radium oppose, Earth and Fire oppose ect.
Just throwing some ideas out off the top of my head. I see great potential here. Have you thought about the use of staves? Staves could be a later game weapon which is hard to build, unstable to all but the most powerful...
Best of luck to you on this.