Absolutely beautiful work. I've been following your dev logs and I'm blown away at how well it behaves on my ~10yo pc.
Here's some feedback...
As already mentioned, a mouse speed control in the options. The default is way quicker than I feel comfortable with and I know just about every player/mouse combination has their sweet spot.
A really nice feature for slabs would be to only have one type/toolbar item for each slab rather than one for each angle. To me it feels like a slab is a slab and it's where you try to place it that should dictate the orientation. So if you click closer to the left of a block face, it should place a vertical slab on the left, if you click closer to the top, horizontal on the top. Should be a reasonably easy thing to make that work... ? It would definitely make the toolbar less cluttered.
In terms of next focus, I would vote for a redstone-like system next. Pistons, hoppers, etc are way less important than repeaters, torches and dust style things with lights, doors, levers and buttons for io.
My next vote would be multi-player, in full understanding that could be really complex, but would be so nice to have.
Again, wildly impressed with this, I look forward to following the progress and contributing where possible, even in tiny ways like posting suggestions.