spells. one thing that I thought about possibly is perhaps making the spells random.
I was hoping I could do something to allow the player to change or have a feeling of progression. but its hard to have player progression without any stats.
Maybe a skills deck? Unlock skills as you progress, allow for three/four card slots in-stage. With this, you may be able to design stages around certain deck combinations.
and the other other idea is throwing out the spells tied to health thing and just giving the player random spells that are only tide to a cooldown.
I liked the idea of the rage meter. If context was the only reason you omitted this, call it something like an "Atomic Meter" or somesuch. As your monster takes damage, its Atomic Energy builds up; if it peaks, BAGOW! Meltdown, death, annihilation, Starbucks goes out of business.
By the way: if you do ever find it in your heart to port this to mobiles, keep us unfortunate Android owners in mind, would you?