I think another thing to improve is the balance between the end game and upgrading ascension dice. By the time you're able to farm the Angel for feathers needed for upgrades, there really isn't any point to playing anymore since you "beat" the game.
I would either: make feathers last through ascensions so you can farm earlier bosses, cap the feather cost for ascension die upgrades, or add more content past Angel.
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I've left the game 'open ended' for that fact.
New ascention upgrades and such could be easly added afterwards. (Not to get your hopes up, as I don't have any ideas atm)
But it would be simple additions.
Removing the feathers after each ascention would increase the game's difficulty fairly in my opinion, that was the reason for trading in feathers for currency.