Spiral Knights has a lot of content, and a lot of variety in level design with bonus rooms, clockworks in general, special rooms like the Tortodrone in the Wolver den. These are all fun and a great part of the experience to see things you haven't seen yet. At some point, you do see it all though.
So what if we could make our own levels? The arcade system already randomly generates over 40 randomized levels every few days, as well as random variety in these levels with layout, enemy spawns, loot box spawns, etc. There are plenty of resources to have a full on level editor system. Something like Mario Maker for example.
I think giving the tool to everyone may be risky with balancing things such if it's to include loot. There's probably a good way to balance it. One idea is to make loot drops based on the amount of level actually created. You could also give it loot options to trusted players in the community, or players who have made a lot of good levels. Surely a tool like this already exists in some sort of form, as the Developers have made levels that aren't randomly generated. I think this could be a great way to do more things as a community, make custom stories, or other fun creative things people come up with.
Some ideas that could be fun are custom hardcore levels with creative ways to balance things like puzzles, monsters, and traps. Perhaps someone wants to recreate a type of level from another game. Perhaps a custom boss fight using little training puppets you can give weapons to. A large custom boss room with the twins, and a Snarby running around that gets stunned by the missiles.
What fun ideas would you create if given the tool?
It's a great idea. That's why it has been suggested many times --- probably as early as 2011. (I haven't checked.)
The randomly generated Clockworks levels, that you mention, are made up of random combinations of non-random, carefully designed "rooms". So consider whether you want a room designer and/or a level designer.
Yes, you want to prevent players making a level stuffed with 1000 treasure boxes. So you hard-code a limit on the amount of earnings. Or you say that player-designed levels/rooms are purely for fun and yield no keepable treasure at all.
And then you have a voting system, where the most popular player-designed levels/rooms get reviewed by Grey Havens and possibly included into the official game.