I implemented a 3d block-based level editor for my FPS game today. It was surprisingly fast, I guess that's because I made a very similar editor a couple years ago.
It supports things like copy&paste and undo/redo (implementation is inspired by rxi's undo method and Dennis Gustaffson's Undo for lazy programmers)
Also apparently people really like the game idea, so I guess I'll have to turn it into a real game! :)
https://fxtwitter.com/jakubtomsu_/status/1794778707854254213