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 in-game texture editor. All changes to a texture and instantly transferred to everyone in the server. There isn't even a need for a save button!