I voted for bug fixing but the more i think about it, the more I'm actually leaning towards a polishing/dejank. Specifically, I would love to see a pass on the UX and general interactions in the game. For example:
Interacting with fired clay items should all behave similarly. Why can I right click to pick up a clay mold with a non-empty hand but I cannot do the same with an ingot mold? Ideally, all items that can be right-clicked into your inventory should be doable as long as there is an open inventory slot.
Filling a bloomery full of iron nuggets is an exercise in carpel tunnel syndrome. Why is only 5 added at a time while also requiring such a crazy key combo?
Filling pit kilns with things like bricks is frustrating because of the first insert adding only one brick, requiring the removal of a brick at the end to make 12. Additionally, I think it would be great if there was a stop point at the pit kiln max if the pile starts in a 1x1 hole to make adding the exact amount require less precision/tedium
Crates are a UX nightmare. Why am I required to hold the item to insert into a crate that already has that item? Look at the BetterCrates mod. Double-tapping right click inserts all items of the crate's type automatically. Coming back from a trip and inserting 12 stacks of rocks, for example, requires me to insert each stack individually, tediously messing with my inventory to do so. This is needlessly frustrating and breaks me out of the game.
These are just the things I remember and I haven't played in a couple of months.
Additionally, the mechanical systems are in need of a lot of dejank. The bug that makes chutes/hoppers/etc spill items all over the place when the chunks become unloaded means you have to babysit your base while you are using automated machines. Defeats half the purpose of automating it in the first place!
Also, the way chutes work (or rather, don't work) to split and merge different pipes could use a look. I think we could remove pre-crafting chutes all together and using the chute sections in-world to build pipe networks would not only be more fun, but also more intuitive.
The quern also behaves in undesirable ways. The most annoying of which is that the operation resets if a side is blocked rather than outputting to the first available side. This makes quern setups unnecessarily complicated and is not intuitive.
At face value, a lot of these issues might seem small and insignificant, but over hundreds and thousands of hours of playing this game, they add up and create a frustrating experience.