Grimrock still has a huge potential for novelties :
out of my head :
Wind Walk / Fly (can be limited by a special kind of resource, like spell component)
Terrain Modification (Lava, deals damage / Mud / Freeze Water)
Ceiling collapse (only indoor)
Teleportation and /or spell of recall
Switch place with target enemy
Most of those spell could be used for new kinds of puzzles
Attacks that deals damage all around the party (Whirlwind or some such)
Spells that deals damage all around the party
Spells that deal AoE damage (3 x 3)
Spells that reflect damage or deal passively when attacked ( ala Shield)
Also spell combination with 2 wizards? Combo attacks with 2 fighters?
spell that weaken enemies (lower armor , speed , résistances)
Big enemies that take more space than a square (2 x 2?)
Or eventually that takes more that 1 vertical slice (think a Dragon in a cave?)
Enemies with weak spots (behind , side )
Or a huge serpent that block a tunnel - the party should defeat it several times in order to clear the tunnel.
I supposed you don’t want to add standard fantasy races? Dwarves, Elves? or some anthropomorphic dog faced ?
traps (caltrops) to weaken enemies
Grappling Hook (as a mean to gain access to new areas)
Walking board to cross a chasm
well , I definitely hope to see a LoG III at some point