Okay, so as a one-off thing (and just for fun - no prizes other than bragging rights) - though it could be done again if successful - here's a new idea.
A speed design contest.
What does this mean? This contest won't be a long-term thing like the normal level design contests. Instead, it'll be more like a live event - we'll arrange a time (to be discussed, so we can find something that suits everyone who's interested) for the contest to take place. You won't have ages to create your level - you'll have one hour to make a level, including playtesting. From there, we have a brief playing phase - this could be a very short thing (an hour or so), or perhaps a couple of days - before voting. The main thing is the need to make your level quickly.
Of course, for this to be fair, (a) the host won't be able to participate (EDIT: to clarify, I won't be able to participate in level creation, I can still participate in level solving), and (b) the rule won't be revealed until the contest starts.
To keep it simple, this would be limited to a single engine. This of course will be NeoLemmix - though if this contest is successful, we can certainly do a Lix version in the future.
Who's interested in this idea? There was some interest when I floated it on Discord, but if we can get an idea of who might like to participate, we can then look at arranging a time.