some people on spiral knights have completed everything and there is nothing else they can do that will challenge them. my idea is to have some thing like the silver key (you buy it using energy form the same place) called "the timer". this would give you a time limit on levels making tier1 things hard etc. but the advantage of having a timer is that any crowns you earn are doubled.
i think i sugested something like this before, but no one actually took a good look at it. but oh well. search it and read if u want.