Exactly what it says in the name; my suggestion is an SK Level Builder.

Whether this comes in the form of a download, an add-on or just something on the website I do not mind.

What reason would this have to be in the game?
The reason I want a level builder for SK is because the GMs have their hands full and alot of people want new depths. A level builder would allow new depths to be created without the GMs having to spend their time on it. They new depths would be judged by a (maybe a few) game tester(s) who would play the levels and decide which ones should be inputted into the game.

But wouldn't people spam it?
Maybe make it accessible for Vanguards only and ban people who repeatedly send the same idea or send nonsense ideas (like a spawn point blocked by wrong-way facing one-way gates). In order for this to be easily done the game would have to link directly with the player's SK account (maybe a 5th interface on the mission menu). Another way to stop it being spammable is reduce the number of maps a person can send per day/week.

Personal Missions
When making the map sending it to a tester is obviously optional. Even if a person doesn't want it to be inputted into the game, so doesn't send it to the tester(s), they may want to use the mission for personal uses. These personal uses could include guild testing, personal training or competition. The competition being in reference to the SK Olympics, where custom built maps built by the judges would offer a bigger challenge to the clockwork races as they become less predictable.

This all makes earning crowns sound very easy...
Crowns cannot be earnt in an unpublished map, because otherwise people would make easy maps (say there's two paths near each other, side by side where the conjunction is split by a set of blocks and there are lots of enemies on one side, but the player walks in from the other (allowing them to shoot hell loads of enemies without the enemies being able to hit them)).

So it's just the normal blocks and mobs?
Unless anyone has any disagreement with that, yes. There will also be location settings. Players will link switches and buttons to gates by connecting them with lines (that cannot be seen in-game). Maybe OOO could introduce some new blocks just to see what users make with them.

Why do you want this?
I want missions to be competed by guildees on completion, not just doing runs for the heck of it. As in, I post a mission into, say, guild storage and it appears in the guild log whenever it's completed. This would make for a very fun event in a guild. :D