Take the existing level editor, provide documentation for it, and build a means of loading levels onto the server and testing them. So I'd make a level, send it to the server using some sort of upload+test option in the menu, and the server would put me in it. I would get zero loot for doing this. That's all that needs to be coded- uploading a file to the server and running it as a level just once, maybe twice and deleting it off of the server. If it takes up too much traffic, limit the number of levels that can be uploaded and ran to only a handful at a time. If someone else wants to test, they'd have to wait.
Why is this a good idea? Because right now there isn't enough content floating around and the players want more constantly. You don't need every player to have an opportunity at creating levels, just a few that are willing to learn and really want to. Then, once or twice a month look on at the best levels. Take those, and put them in the game. Just like that- a truly infinite world begins to build itself.. and if that wasn't the intended nature of Spiral Knights then I don't know what is.
I just replied to another thread that you said this before, then I saw this. xD
Although, yh. +1