After unlocking Root and Luna and reaching the two 100% progress final bosses, I can say that I enjoyed the different tactics of later bosses which made them distinct from each other.
Here are some ideas:
Pray To The Goddess: It's OP with Rosary since you can spam it every turn if not Cursed and only take 1 dmg from every attack. Only Poison, Bleed, and multi-hit counter it.
So maybe on higher difficulties, you can only Pray 3 times per battle and have to pay 10 Lunar Fragments for every subsequent Pray?
Challenge character:
- Starts with 6 temporary hearts.
- Starts with Knife
- Starts with Gun.
- Starts with Second-hand Smoke boon.
- All max hp gains/healing items/rest events grant temporary hearts instead (cap of 6 gained from percentage healing).
Two deals with Living Guts basically allow you to easily kill final bosses. Maybe reduce its effect to 1/4?
Doomed/Despair status effect: Increased by 1 by certain enemy attacks, such as charged attacks, and lowered via Pray/defeating a boss/certain events/items. If the counter hits 13 (carries over between battles), explodes and take damage equal to 1/3 max hp.
All bosses so far spend a turn to heal but what if later on, they had a life drain attack (dmg-based healing)?
A boss whose Charged attack immediately resolves all your poison stacks dmg at once, punishing you for ignoring and not curing it.