Not sure if this has been mentioned before, but in Chelsea's newest message "Truth and Beauty" there is the line:
"On the clocks atop the citadel beyond the shore"

Citadel is one of the "epsiodes" in Marathon 2. It houses four levels, unlike some of the other chapters which contain only 3, or even just 1 level. We've already seen a Marathon 1 level name used (Arrival)... perhaps this is some other kind of clue? The Citadel in Chelsea's post is such a central theme (it seems) that it just seems to me to be too big of a coinicidence. Then again...

Something to think about, I guess. I can't find the picture displayed at each new chapter, perhaps there's something there, too.