Caveat emptor: There will be no trog pr0n here, so if that's your predilection, please move on 
Just like units lack any sort of real differentiation due to almost no special abilities, magic is stuck in a similar rut.
I'd propose that at the very basic level, targeted or AoE damage spells need to have well defined secondary effects, at least once you get beyond the most basic ones. In the following examples, I'd suggest either X or Y be determined by shard ownership (most likely Y since that's indicative of scaling damage), though probably not both since that would fall into the trap of overpowering shards. Assuming that it retains at least a similar mechanic as was intended and not currently working in game.
The first entry for each will be single target the second AoE.
Each entry starts with "X% chance to"... which I removed in v2.0 of this writeup since it was getting a bit overdone.
FIRE
Set unit on fire for 3 turns doing Y damage.
Set each affected tile on fire for 3 turns. Units take Y damage passing through or starting their turn inside any of said tiles.
AIR
Knockback and attack/defense reduced by Y for 3 rounds.
Make each affected tile "windy" for 3 rounds, reducing the attack and defense of any unit while they remain inside.
Earth
Damage turns into "true damage".
Affected tiles become "difficult terrain" for 3 rounds, thus requiring extra AP to bypass. Or perhaps becoming impassable.
WATER
Freeze unit for 3 round, small chance to shatter for Y extra damage at the end.
Cover tiles with a frosty rime, 25% chance to lose remaining AP when entering.
I tried to keep conflicting and overlapping ones to a minimum, and realistically this is just some off the top of my head spitballing.
But the beauty is that they could be tied to magic research or initial book choices (school specialization?) as well as shard ownership, giving a fairly decent level of customization depending on your playstyle.
