We think swapping out your spells is a neat idea with plenty of potential for growth!
The improvement we'd like to see the most is to have visual and audio feedback when hitting the boss and when getting hit! It would also be neat to see the bosses have more attack patterns, most of the time they were running away from us.
It's neat that you have a hub, and we think it fits a Boss Rush game quite a lot! It's nice to have a place to rest between fights.