The ability to counter any or most spells sounds very interesting, and I sure hope that something like this gets into the game...
I've always loved Steven Eriksons take on magic - basically, its dangerous. Very dangerous, and capable of destroying armies at its highest levels, but also very taxing, and dangerous for the wielder. You might end up being consumed if the power you try to control, slip from your fingers.
I think this is something that comes back in other Fantasy novels too, I'm thinking about The Wheel of Time here. channeling the One Power is dangerous, trying to use to much power can hurt or destroy the one channeling it.
This I would like to see in the game as well, perhaps as a way over getting more power out in a desperate bid for survival, you could up the power of a particular spell, with the risk of dying, or at the very least suffer a severe setback (loss of abilities for a set period of time for instance).
The spell itself could become stronger, cover a greater area, hold for longer time and so on.
Every spell could have various power levels, higher levels can have a bigger chance of fizzling out or have an unintended consequence. For example, a normal Fireball spell does 200 damage to a target. The second level does 300 damage, but the range is less. The third level does 400 damage to a target, less range, and also 100 damage to the caster. The fourth level does even more damage, but has a chance it will backfire or fizzle out.
I used to play a rogue-like game called Angband. In this game a mage could cast spells safely as long as the mage had enough mana. If you tried to cast a spell with insufficient mana, you got a warning, if you tried to cast it anyway, there was an increased chance it wouldn't work, or that the caster would get unconscious for a few turns. Something like this could also get implemented in Elemental. If you're losing a battle, you're almost out of mana, in a last effort to turn the tide your hero cast one final spell... and the enemy is defeated. (Or the caster is turned into a rabbit...
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