... Open a Steam Workshop.
(Warning: Lots of words and guess-work ahead. If you do not like to read, there is a TL;DR at the bottom)
Now, I know this has probably been mentioned a few times throughout Suggestion threads and brought up plenty of times, so before you all rage at the bottom about why this wont work, I'd like to explain myself.

I've read many of the responses from admins on this forum (not on this particular subject, just other general topics) and they seem to be stretched thin. They struggling to make updates, they struggling to maintain forums, to respond to bug reports; everything dealing with this game. I can tell these guys work very hard to make all their players happy, but OOO just has too many demands from its players and too little time (and workforce) to meet those. We all want the developers to move onto the big things, like opening up the core, but they need a little help to do that. Of course, taking weight off the devs isn't my only support to this. There are a few other reasons

- Players want new levels.
Since the removal of energy requirements for the lift, many of the people I play with got really bored, really fast. Now, we all know the little prompt on the lift saying that we would never play any level more than once, but that simply can't be promised. They did give it their best shot, though.
- The levels could be used to make the community more involved with the game.
'' What do you mean by that? '' Well this is what I mean. Now, I'm sure there will be quite a few level designs posted once the workshop takes off, so here's what OOO could do to help filter out the good and the bad, letting their players contribute to the game: The levels will be uploaded, but not implemented into the game until they have gone through voting. I would say that the top 10 levels would be reviewed by the developers and once they're approved, they could put them on the beta server for players to actually test. Which brings me to my next point:
- The E Market just might stabilize!
To gain access to the beta testing server, people have to buy about $5 USD. Once it's seen that you could be testing once a month on levels designed by fellow players, I think more people would be more motivated to buy energy packs. Another thing to consider is that some of the levels that could be designed may be too large to just slip into the game (i.e. is large enough to stand alone as DLC.) I've seen some people make very cool things with the workshop, and I know there are a few special people out there who could pull this off.
(By the way, if this becomes a thing and someone could design levels, someone please do this: http://forums.spiralknights.com/en/node/87661 )

As I said before, by opening the workshop, OOO would have less on their plate. No more having to worry about making new levels for players, no more having to throw silly promos every two weeks to get players to by E, and no more having to worry about fixing the bugs or upping the spawn rates of things because of the promos. After the shop is opened, they can just focus on programming the opening of the Core (and hopefully new weapons and gear.) I know they'll still have to evaluate those designs, which is still work that would be taking away from the Big Picture, but if you look at the long-term effects, it would save so much time.

For the tired and/or brain-dead:
TL;DR:
1. Open Steam Workshop
2. Evaluate the top 10 voted by the community, & put whichever ones are approved on the Beta Server every 1-3 months (preferably 1)
3. Convert projects into new levels, new prestige missions, and/or new DLC
4.???
5. Profit.

As in any thread, leave a comment describing why or why not this would work.