Imagine if you combined Garry's mod with Minecraft, only better. This game will have an in-game editor, so you can make changes on the fly, and have your players see them immediately. Need some trees? Just open the editor, draw a quick texture, make a new block type, and voala: Trees. No need to send .mod files or restart the server, everyone already has your changes.
This is the current editor for block types. We decided to make granite look like stone slabs, and added a new block: mud. Thanks liquid checkbox.