Four months and a mega update later and this game is still fantastic! (Also the new weapons and items are just great- especially the new 8-cost ones, though that may just be because of how hilariously strong they are.)
In terms of the unlocks and new game plus thing: This might literally already be a thing once you beat the boss now (I don't know, I haven't managed to get back to it in the newest version), but what if the new game plus was more... expansive? Instead of just four fights, a chest and a shop, what if it had a rudimentary branching path system akin to other roguelikes like One Step from Eden, and each world had a boss? (...though this would be such a significant change that it's probably going to be reserved for DLC or a potential possible sequel, both of which I'd also be excited for.)