One month is 4 weeks. That is enough time for 2 big events (2 weeks time) and 2 small ones (a week time).
Here is an example of monthly themed events:
Theme: Sci-fi (Science Fiction month)
Big event 1: Battle beyond the stars - tournament which phases are organized on different planets. On each planet different mod is used with different gravity settings, also as tournament progress the number of damage points total during matches dictates some special conditions. Players are set into 3 groups (human, alien, robot) and each group has some advantages and flaws...
Big event 2: Dangerous Planet - another tournament event where players are set into 2 groups. They duel each-other and post their replays. Number of points winner has made is converted into kilometers with special mathematics formula. It is visually presented as two color lines that grow bigger as the number of passed kilometers adds.
Two groups are competing who will explore the most of the planet's surface. Team that wins get prizes, while those with most scored wins get some bigger prizes etc...
Small event 1: Build a robot - Basically a design event where ppl are making design of the robot with specs (height, weight, purpose, tools it carries, overall look...).
Judges decide who had the best design... It can be blueprints or 3D image or drawing, as long there is a detailed description of all of the robot's parts.
Small event 2: We landed on some strange place... - Another design event where ppl can make photo manipulations or paint an image of landscape of some alien planet... Judges decide who was most creative etc...
You need one banner, to assign your GMs who will lead each event (2 for each big event and one for small ones + judges they can take from community if they wish) and some time (15-20 days) to prepare these events (text and some images, but nothing too much...). While events are ongoing you can assign some ppl to prepare scenarios for next events for next theme, and so on.
If it's too much you can even do one big and 2 small events each month.
I sat and came up with these ideas in 5 min, I'm sure a team of 10 GMs and trials could do better and much more.