Came up with this in the morning.
Rule: you can only cast a spell in a battle x number of times in a battle.
X is your spell level / level of the spell.
If you have spell level 4, you can cast a lvl 1 spell 4 times, lvl 2 twice, level 3 or 4 once.
lvl 0 spells have no limit.
To me, this would encourage researching a breadth of spells, not just spam arcane arrow/spell blast/touch of entropy.