I'd say this task isn't really all that complicated.
You've got the interface and the actual terrain construction parts with tools and stuff, (maybe you can find a FOSS minecraft one or something) and you've got the
utterly trivial compatibility part with .blb's and stuff.
I mean seriously - you could make the system read .blbs for complicated custom stuff like JVS doors and stuff, and instead of loading .dts, just have it make a normal solid brick with the same dimensions as the final thing, and use it as a dummy.
You could pretty easily make a full on brick building thing like that lego one with a blue brick icon that was out a few years back.
The vast majority here will be aware that two base engine classes, Terrain and Interior, were removed from Blockland in August of 2014.
i'm sorry but they what now