Can Unity make a map editor (say i made a game and wanted to publish a map editor with it)?
Sure, there is a whole terrain editor made with unity and there is the road creation thingy, also pure unity.
What you just have to keep in mind is that you have to write saving and loading routines for your map format as you can not create unity levels and alike from the player
also if you allow them to add new media, then you potentially will need to write loaders as the unity classes are targeted at loading existing imported / asset bundled media, if it isn’t music, image, video or text where WWW offers such functionality
This project has a basic level editor included with it which might be useful for learning from.