Looks nice! You have a pretty neat voxel editor here!
May I suggest to support
Minetest Schematic files too? Here is my own
spec on the format, maybe a little bit more readable (if the resemblance is uncanny between these two, that's because they have used my spec as a source for their wiki ;-) )
And maybe voxel
Model3D (Goxel2's native format, it is not using .gox any more). it could be a good candidate for storing animated voxel images, if you finish implementing animation.
I've also noticed a small bug
in the doc: you say that vengi supports .nbt, .schematic and .schem files, but you link to the same spec for all of those. However this isn't the case. NBT is an block encoding method (reinventing the good old chunks), and both .schem and .schematic files use that chunks format, yet they are conceptually different. The link you use is for .schematic files only, and the best spec I could find for the latter is
here. Using the same link for all these three is pretty confusing.
The proper links would be
.nbt,
.schematic and
.schem. Hope this helps! Otherwise well done!
Cheers,
bzt