As has been revealed in the contract between Bungie and Activision (made public earlier), assuming that Bungie's Destiny makes it to market and certain revenue goals are achieved; a portion of Bungie staff will be allowed to work on a project called Marathon. We here assume that if Marathon is a code name, that it represents ... Marathon.
One can only imagine what that game would look, sound and play like, given what we know about current and near future game engines and assets. Some wonder if it would be a remake of the series we know and enjoy, or another story based in the Marathon universe (such as the player being one of the remaining 9 cyborgs left on tau Ceti during the time of Marathon 1). Just an idea.
Either way, think back to the art work which graced the Trilogy's splash and chapter screens. With most of that being done by artist Craig Mullins, imagine what such a game might look like if his artistic hand took the reins of such an endeavor. Of course there are no guarantees that such a thing would happen. Also, Bungie is brimming with artistic talent to bring to the cause, so no matter what, it's bound to look fantastic.
I for one hope that Mr. Mullins will be the one to lead the artistic effort; in a sense coming full circle and his vision guiding the direction of the look of the game.
Perhaps co-creator Jason Jones (and who knows, maybe Alexander Seropian?) is seeking some kind of closure with such a project; bringing Marathon fresh to the masses, while ensuring that the community that first embraced and supported Bungie is not forgotten in the process. One can wish!