I actually originally planned to develop Rubicon as an exercise in intensity. One of the problems I saw with existing scenarios was how a relatively uncomplicated plot and a restricted amount of new artistic content was stretched over a large number of levels. I wanted Rubicon to recapture the intensity of the original Marathon games. The initial plan I came up with involved all new art, maps, story, etc crammed into twelve levels. Since then, the details of Rubiconis goal has changed a lot, but the overarching desire to make the next marathon game has remained the same.
Marathon Rubicon is currently in beta, and the availability of the final version is not yet known. You can find more information at the Marathon Rubicon Web site.