If we're not going to get a level editor, then I might as well suggest this instead. Next best thing is still new levels.

A level based on pickups (U76 suggested this a bit, and I'd like to further it a bit)

I think that it would be really nifty if OOO made a level based on pickups like vials and barriers. That is, had them infinitely respawn, and throw a variety of situations that warrant using them at us.

A digging level

We often find groups of starblocks in the clockworks, or groups of various things that we "dig through", but there is currently no level that has a ton of concrete slabs for us to tunnel around in. I think that this would be really fun, and could be a cool way of using the monster blocks. The level would presumably have a layout, so as to not just be ALL tunneling about. I.E. Walkways running through it, and a general path like on other levels-- the difference being that you could potentially leave and tunnel off a way.

It would also perhaps have some kind of explosive pot type thing for bombers and people with slow weapons to use on the blocks and tunnel faster.

Revamped Arenas

Currently, there is one arena layout in the game. Yes, there are many arenas of many types and shapes and sizes, but the arena level found in the clockworks could really use another layout, or some level to replace it randomly. The idea is simple: Make another level with different enemy waves, but under the familiar fun high paying arena format. Maybe even put mini enemies into it, and make it like compounds in theme but not payout.

A straight path to the exit with 20 danger rooms on it.

You have no idea how much energy I would spend on this level, or how many times I would replay it. Maybe even make two or three of them free and hide buttons in those.

And yes, I did mean cumulatively on that level. I would spend exactly 60 energy on the danger rooms each playthrough.

A level with a massive number of buttons.

It would be a large plane, and would contain nothing but a field of buttons. The goal to this level would be to press the buttons, and doing so would spawn enemies or treasure, with one button opening the exit.

A level that abuses the one way gates.

A large maze like level, with many one way gates and other puzzle like elements- statues, keys, what have you. This level would offer options on where to go, and would be randomly generated with segments just like in the arcade.

A level that abuses the height mechanics

It would have its ups and downs to be sure, but the entire thing would be built on a hill. The knights would advance uphill into various turrets and what have you.