While I'd read in the other comment thread about anime shows vs using the site mascots, I'm making a new comment since it could develop into a deeper side discussion.
I'd like to stick with using the mascots because, if selling the calendar becomes a thing, it's completely original content and not fanart. Instead, if we went for a unifying calendar theme that people voted on, it may make this project more dynamic.
The last artbook I was part of, we kept the themes based "earth" and had color themes as well. It varied from nature to urban settings, but it evoked the concept of living life. Having color sections harmonized the artbook, so that it transitioned like a rainbow.
While CL is "colorless," the mascots can remain in neutral forms of grey while the scenes for the pieces in each month focus on seasonal colors. Winter, spring, summer, fall. From cool to warm colors. From there, the artists can interpret the months based on how they feel inspired.
But of course, these are just ideas. I'm interested in seeing what others have in mind. I've never seen the other calendars before, so I'm completely unaware of what has been done in the past.