I would have them fill out what we have before going too wild with tangents.

Missing items... Stuff like blackwood walls (full, half, 1x1) and floor tiles, 1x long beam, slant beams, normal sized door, etc. Stuff like black metal hoe, pick, core wood construction hammer (even if the only effect is massive durability increases and minor cosmetic (reskin the metal). Maybe a 2x2x ?? prefab chimney tube.

Missing or incomplete concepts.. Stuff like being able to turn some kinds of lights off and on (fuel consume stops). Being able to put anything on the item racks. Nail down how they want to handle comfort items. Double distance for upgrades to bench/forge/cauldron/etc.

Next biome and boss. Even just 1 would be huge, probably mistlands (?) or finish out the ocean ideas.