Please don’t vote until you have read the whole of this post. The reason is that the question itself needs some clarification.
Would you prefer a new game or a new mod for you favorite game to be released every month?
First of all I need to clarifiy exactly what you are getting each month. If it was a mod then it was a mod of substantial quality. Something like Coastline to Atmosphere for Half-Life 2. It would always be a single player mod and be for your favorite game, not just Half-Life 2.
Now for the game. It certainly wouldn’t be a triple A blockbuster like Bioshock or Half-Life 2. To ensure a certain standard let’s say that it would be as a good as most SP FPS games released in the last few years. Each game would introduce a completely new storyline, weapons, enemies and environments.