If you are directly casting, you'll grow fatigued; rituals, however, might just depend on time, resources, and other factors. As I said above in this long reply, magic will be very strong, but very rare. I do like the volcano-raising idea! Possibly none will be of quite THAT power... but, then, maybe they will. We'll just have to see! Chances are, big battles are going to be especially big, so magic is going to have to scale.
I have a rather... complex magic system thought out but i think it will not be that suitable to your game because it would shift the focus of what you're trying to do.
Well, if your wizards can split the earth and swallow entire armies, why bother with an army at all. Just recruit a bunch of wizards!
A mage need something between himself and the sharp pointy objects that others want to put in his body.
As long as a mage can undisturbed unleash is powers it would be a one sided battle, unless the other side have some nullifying capacity.
When both sides have mages it becomes less flashy as then the mages job is to nullify/unravel/counter the other mages spells and if they're equally strong, no spells will be cast at all.
Blaze mentions Channeling ability, which sounds like a good measure of a mages strength. As this is a rougelike i assume that actions is measured in ticks.
Example 1:
You're a crappy novice mage, your ability is at 1 and you want to throw a firebolt in the face of that approaching bandit.
But firebolt costs 3, so with just 1 in ability it will take you 3 turns before it fires.
Example 2; Mage duel:
Mage A got 20, Mage B got 22.
The mages elect to dedicate their power to counter the other one, But mage B got 2 points more than A.
Mage B is then free to use those two points to cast spells.
Does this mean that Mage A is screwed? That depends on other factors such as his army being larges and can take the casualities, or Mage A got items he can draw power from, a ritualistic summoning that only needs knowledge and no channeling.
Maybe A Isnt that strong in raw power, but is trained in counter magics, i.e. 20+4 vs 22.
My own little system is based on the theory "power is drawn from an aspected source, limited by ability"
Sources would be realms/domains which is connected to the mundane world in different ways. Such as the elemental domains being intertwined visibly with the world as fire being strong in a desert, water on and by the sea etc.
Divine magic is limited to what the caster's god is willing to give.
Necromantic magic is drawn from living beings, where blood is used as fuel. The potency of the blood would be based on multiple factors such as health, age, magical traits or being a sentient or not. Drawing power from the dead and dying on the battle field would just yield a fraction of what a controlled ritual draining would.
The different mages would fall into the categories of;
Innate, where the mage channels his own power reserves, a quick but corporal fatiguing way of casting. An elementalist born with an affinity for fire would use his power as a conduit to manipulate the fire realm, drawing upon heat in the surrounding.
Knowledge/Ritualistic which is the use of arcane symbols, letters, language, geometrics, to form a spell, a target and the conduit.
And i got sidetracked. not much of this is useful in game mechanics.
Simply summarized. Channeling ability which determines the speed the mage can draw from a power pool, innate or external to fuel spells