Take a look at this chart.
http://steamcharts.com/app/99900
Then hit the "ALL" option to reveal this Spiral Knights's two-year Steam playeroom entry flow.
There's a remarkable pattern here.
The Sprite update escalated the player count. Completely.
To make this again, mechanics need to be added. The Sprites were a nice mechanic (who doesn't like the sound of pets!?)
Make something that sounds nice. Make a new expansion. Make people grind.
This isn't really a suggestion, even though it includes some. It's more of a thing to realize a pattern in the player poll and how things affect it.
Also notice the steep drop in players. During the summer the slime casino helped even out the line, but content felt short-lived.
Hopefully this is read by the staff. I understand the staff is small in number, so I don't mind waiting.