: At the time, they weren't planning to make a third game though, IIUC.
Nope. Check out.
http://marathon.bungie.org/story/newmar-apr98.html#Marathon3
There was even a plot treatment for Marathon 3 according to Matt Soell. Matt went to some efforts to find it. Something about a pyramid map and a submarine. In the end he left Bungie before finding it. Most likely it was snaffled for another game.
: Marathon 2 was going to be The End of the story, so they could move on to
: other projects like Myth and Halo, and thus they wrapped up all the loose
: plot threads with a nice little bow (well, other than what happened to the
: player character—he was still stuck as a slave at the end of M2). Having
: Double Aught do a third game was something the decided later on.
Yes. The final screen of M2 tied up a number of loose ends regarding the Pfhor, the S'pht , Leela, Tycho's clones, the surviving colonists. But more importantly it allowed Durandal and Mjolnir Recon number 54 the opportunity to travel to another ruined world this one orbiting a rogue star. No doubt this world had a hole though the middle.
Cheers
Hamish