Really love the game, I think that a few features would really add to it, but it's really, really good.
The ability to loop after beating the final boss, keeping character upgrades and items, while adding a curse for every loop would add another layer to the game, and it would be nice to be able to play longer, upgrading every hero to max.
The ability to skip or reroll hero upgrades. This is probably something you've seen a lot, or are already working on, but sometimes I have gotten a build going that works really well with a level 2 hero and I'm forced to upgrade them. It's not run-ending, but it's annoying.
The game definitely needs music to feel complete, I don't necessarily mind the lack of music, but the it being missing is one of the few negative points the game has.
Something I think would add a layer of strategy to the game would be the ability to swap hero positions(Example: moving thief from the first position to the third). This could be done between combats, or only after bosses, only at the start of the run, or any other number of ways. There are a few things that affect certain positions, and this would let you play around those more.
That's really all I can think of right now, the game is already really solid, and super enjoyable. Thanks for all your hard work, I'm looking forward to seeing what's to come for Slice & Dice!