Signs
I'm sure someone must have thought of it, but I'd like to bring it up again; signs would be absolutely wonderful to have in a player created environment. I imagine something like a signpost where you could put a board pointing, say N, NE, E, SE etc. and it'd show only the placed board(s) on the actual post, with the text in the target box (even better would be actual writing on the wood, but I'm sure that would be too technical/demanding). It'd make the world more lifelike while allowing players to get more in depth with road naming, city naming etc, all while also make the world more friendly for newer players and generally easier to navigate.
Markers
Markers to aid construction would be really helpful. I envision something simple like a stick hammered in the ground, perhaps even coloured. It should be easy to make, place and remove again. To really make it worthwhile, I suggest you could put it at a certain depth to indicate the ground level you want to reach, and terraforming should then be possible while the marker is still in the ground. This would make it way easier to plan ahead, not to mention explaining a group of people where/what they have to work.