I like your idea, though I don't think fresh water would be a realistic resource to get from Mars. Rather, I think desalinization plants would be used to convert salt water into fresh water, as they are used now. The only reason we'd need to get water from Mars is if all the oceans dried up on Earth. However, if that happened, we'd all be dead very very quickly anyway. I don't mean to put a dampener on your story, though!
Perhaps a rarer and more materialistically valuable substance (ie precious metals) could be the motivation for the first expedition.
Cora Miles would be a good character to reintroduce, as she was a candidate for Governorship or some such US governmental position in the original story. Perhaps she could be the one who organises the mission. Of course, Ken Borden could be the pilot of the rescue mission.
Will the Cocytans be involved at all? Are you personally making this game, or is it just an idea for the future? As you're probably aware, making a fan game these days is frought with legal difficulties!