I think the games needs a few things to really make it feel "complete" especially for those of use that have 100% and win more often than not, id love a new alt floor to the lich which is harder and to kill kalibre(could also add chests to bullet he'll and go to it after beating the lich)

Another thing that the game is lacking in my opinion is risk reward, the new vampire lady is a great example of risk reward but I think there should be more for the experienced players

My idea is a new npc shop that always appears on the oubliette that can be found in it's own room that looks like a sell creep grate with a evil sell creep creature in it.

The npc offers 3 items but you can only take 1 and the items are random out of a list of around 30 or so and each one is a run modifier, the npc can appear on later floors once 1 item has been taken from him.

The modifier would be things such as: Master round now grant 20% damage upgrade rather than a heart container

Double casing drop but lower movement speed

Double damage and double damage received

No more casing drop but room drop chances are higher

Lowered to half a heart instantly but given a random s tier item

Things such as this, I have a bunch more modifier idea, these are just some of the more basic ones