How about adding physic to constructions, by adding some gravity, weigh and strength to material, so for larger scale building constructions on planets, players will be forced to recreate a stable structure to build on.
Making impoosible to build bridges without pillars, suspended houses with a triangulated structure, or even a floor without a certain thickness, neither a dirt tower....
As i understood Voxel builds is composed by millions of tiny tiny pixels cubes, so it might not be difficult to make calculation to make cunstruction horizontally collapse based on distance of last cube from attached solid point.
Here is what we know about Voxel:
Here is what i m talking about: