Former Blizzard Employee here:

We used to have this thing called "Show And Tell" where full-time Blizzard employees would get to see what was coming along the line. We were shown pre-alpha stuff, including rough gameplay, art, story, concepts, etc.

The story of the game was set on an alternate history Earth in the near future. Aliens come to Earth and crush humanity. In the closing days of the war, humanity figures out how to defeat the alien force. Using time-portals, hundreds of "jumpers" - people who can bend reality and "jump" through space/time - go through with the information to defeat the aliens. One makes it through and when the aliens come, humanity defeats them pretty handily. Players were supposed to pick up years after the second timeline.

The game was a sort of MMOFPS with classes and an "agent/citizen" mode. When I went to "Show and Tell," three classes had been fleshed out and presented. One was a big warrior guy who wasn't very memorable. The other two were much more interesting - the Jumper and the Reaper. The Jumper could teleport around and used pistols and knives to kill, while the Reaper had a seemingly unlimited supply of guns, tossing empty weapons aside a la Matrix tower raid scene. Gameplay was going to occur all over "Earth" with really neat, region themed enemies - e.x. tribal themed mecha, robots, and enemies in Central Africa, while Mexico enemies had a much more Aztec feel.

One of the more interesting ideas about Titan was that there would be two modes: Agent and Citizen/Civilian. Agent mode is where you would do all of your combat and missions, while citizen/civilian is where one would socialize with other players, run shops, etc.

The game looked really great and we were all reaaaaaaally excited about what was coming along. After I left Blizzard, I heard a rumor here and there about less information coming out on Titan, until the recent cancellation.

Cheers!