1.Idk if u have thought of this but maybe a pvp? where the arena will be bigger than the sandard arena and the players can just use whatever skills they want?
2. maybe a creative mode? where u can draw ur own monsters and animations with an editor and maybe give them abilities from the player's arsenal.
3.Like have different ability types? Some Support some Defensive some Aggresive (this could work with lets say W being ur attack spells and Q being ur support/defensive spells.
4.Leveling up spells. I would either see this being like u find an area with a chest and if u keep that chest "Alive" until the end and then u get some sort of token to level up ur spell or when u kill enemies u get coins to upgrade ur spells
5.Add companions that would act either with them being at the side giving u buffs or being in the arena fighting with you.
6.Maybe instead of buffing basic attack u make the player able to unlock new basic attacks kind of how u unlock new spells
7.In some levels u can save a "shopkeeper" from some enemies and if u succeed u will be able to buy spells that u want from him
8.Maybe arnor? armor that you would apply on your character to give him/her buffs like regen 10 health on hit or take less damage
Thats all my ideas for the game that i feel like arent too op or too dumb to be added