When there are more mods and stuff... this will have to be something supported by mods.
I'm going to primarily concentrate on making the role playing game. I add some simple things to the pure build mode when it's easy or really necessary... but an in-game editor is kind of a lot of work.
Besides, at some point there will be building templates and ways to place them and size them... this might be similar to a world builder. When the NPCs and the world generator generate buildings it will be based on a library of templates... shapes, architectural styles, etc.. It might be possible for me to give the player some control over that.
And as far as external tools to edit, that's one of the reasons I created the external tools section and posted the Java notes on how to connect to a world database... because another player was asking me about it.