Sometimes they add new maps to the game. These new maps will be part of the dev picks / voting for the next rotations. This wont happen every month. I think this happens 2-3 times since release. Maps like Crater, Volcano Islands, Michi (for team games) are examples of these kind of maps.
I would love to see some competition for monthly new maps from the community that makes it into the base game. These maps could replace the dev picks for example. This way you can have 1-2 new maps every month (1-2 in each map rotation). After the month, you can put these maps into the voting.