Level 100 Moves:

I believe that level 100's should get a big reward that could be shown, my idea of this is a upgrade of moves
But to make it less unfair that ALL 4 moves get upgraded the player must only choose 1 of them to upgrade to tier 4 giving them there last decision in the game

For example:

Ice Snake could take up more space (3x3)
Multi Hit could surrond the player instead of being in one place
War Scream Could send small hard hitting shots with each giving a effect to the player
Bomb Arror Could Launch 3 arrows (just like in arrow storm)

If Anybody Says: This is To Op Its Freaking Level 100 bro ITS MEANT TO BE SLIGHTLY OP

 

but then there would be no reason as the only thing you'd use level 100 spells for is getting to level 101

 

I like the idea of an upgrade where you can only choose one of them, but if you really want this, you need to add more details. Ex. creating upgrades for all spells, specifications on stats, etc. There is potential here; Refine it!