I like to be creative. I have seen other online games that let you make maps and other stuff, so I thought, "It would be fun to make a mission for your friends to play".

If you are interested in this idea then consider these advantages for Spiral Knights AND the player;
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Spiral Knights Staff (ect.):
- You can gain some money off this. You can make players pay for making maps, after the first three they make. (which will be free.) Besides that, you can publicly announce and attract the creative gamers crowd to come out and try making maps. A.K.A. more members, plus publicity, which you seem to be looking for. If you made this game for people to enjoy this, and for you guys to get some cash then you would deeply consider this suggestion.
- It would get older users who don't get on often have the urge to get on more, if they hear about it. I mean old users would do a lot more since they haven't been on for a long time. They're basically new members.
- It would make you one of the best MMO's out there, because I personally think that it's the only thing you're missing, maybe besides better health vials. (That don't leave you after the mission.)

Players:
- Players would have fun making levels, even though it would take some time, it would be worth it and they could see what there friends can do.
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Making it could be something simple like Gremlin there, person here saying something, treasure box, make them invisible and when a monster dies just set it to if (Monster name) dies make (Monster name) visible so they can fight, and also put a limit to how many things you can put on the map, like have treasure boxes 25, enemies 200, and People 30 (The guys who you press Z to activate). But make number of tiles limited.
Also you should put that there is a certain minimum amount of each object you can put in each game, which would get rid of players putting just boxes in. (I win)

It should stay nice and simple, but still as good as if they were a good programmer making the level on an game maker.
I hope you consider this, because it would be to everyone's advantage. I think this would be possible, and even if it took long to make it would be worth it.