I’d love to have 0.5x0.5x0.5 blocks. The space of one block would fit 8 of these and you could place them however. For example, you could make an L shape out of them that’d be like half a stair. It’d be amazing for building small things like furniture or detailing buildings.

Vertical slabs are another one, obviously, but also mixed double slabs, horizontal and vertical. Like you could mix a horizontal brick slab and a horizontal planks slab and make a wall that is brick on one side and planks on the other, all while being only one block thick.

Another one is having a slab, stair, wall, door, and trapdoor of pretty much all blocks. Wool stairs, for example, would be amazing for furniture and those carpeted stairs you sometimes see irl in fancy-ish places. Doors/trapdoors would be awesome for secret passages and super thin walls.

Also a built-in way to make customised worlds, like single biome worlds, worlds where only a few selected biomes can generate, worldpaint, and such.

Oh yeah and a colorful poisonous frog that is smaller than other frogs, spawns in jungles, and gives you poison and nausea if you hit it.