Hmmm before release I did consider making other things unlockable (besides palettes), but ultimately decided against it since it's a small game anyway so it would feel kind of limiting. I might add like, a new game+ in the future when you beat the boss, but until then I think I'll just stick to palettes only for unlocks:) Also when I said I fixed the window issue I meant in my version of the code haha I haven't uploaded the updated version on itch.io (there's a lot of stuff I'm in the middle of adding), so for now you'll just have to bear with it, sorry about that!
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.)
I'm glad you're still enjoying the game this far after release!:) Haha I gotta say, that would of course make the game more interesting, but also would be a tremendous amount of work for me, and to be blunt, isn't gonna happen. I do have... something related to level/world progression planned for a future update, not quite as complex as what you're describing, but I think it'll be interesting all the same!
ooooooooooooooooooo [more audible excitement noises]
also while on the topic of future content, I just have a couple little suggestions for new items
weapons: one that bounces off walls/enemies (but is just a single moderate-size shot), or one that splits into two shots going left and right when it hits an enemy
items: one that makes blocking easier, and one a la mirror ammo that makes blocking recover health (at half the rate ammo would be recovered normally) INSTEAD of ammo