(Yes I know this suggestion came across already, but this is my vision)
People enjoy customization, I have seen it in numerous communities from many games.
I think that something like this would offer much enjoyment to the game.
I think it would be fun if there was a system that allows people to make their own missions and levels.
These missions would not offer prestige, rewards, anything and (actually) shouldn't cost anything, as they're just here for enjoyment.
However, it might be fun if there were achievement rewards. For example, if people played your missions enough times you'd get a not tradeable costume hat, or maybe a special weapon based on what rank your players are. A brief example.
("An example in an example, Juv, you're going mad!")
When level-maker is recruit. Players and below have to play your level 50 times together for a 1 starred weapon.
When level-maker is vanguard. Players and below have to play your level 2000 times together for a 5 starred weapon.
The missions would be playable by any player. The depth will be calculated like the way you adventure through prestige missions.
It doesn't have to be 'block by block' customization. More like the common roads and battle spots from a clockworks level, but with still holding the feature to place breakable blocks to generate obstacles and increase enjoyment, as well as a feature to set mobs on multiple locations.
The mission/level needs to be tested first(without a gun, due to some obstacles requiring pots,) before they become available to the public. It will be possible to test it solo, with your friends or three spiral knight bots.
The missions would be able to be set single or inside a collection.
The missions will be available in a special "Custom" tab next to the current four tabs. The custom tab would first off show your current downloaded list of missions, a recommended download-able mission and a link to the list of missions which are able to be downloaded. Steam users would be able to download the missions through the Steam Workshop as well. You will be able to rate the missions as well. With either thumbs up/down or a 5 or 10 star rating.
I know this idea is big and difficult to add. But then again, it's just an idea and I thought I'd mention it.
"Players as vanguard have to play your level 90 times together for a 5 starred weapon."
Um...so players get items without needing to spend CE, the main source of income for OOO?