So it seems that the only interesting thing to come out of PAX '08 is a picture of a TV with the main menu of a possible new Halo game.  The folks over at AddictGamming.com managed to score this picture while at PAX.  The interesting note from the screen shot is that there is no campaign option and it's a later build of Halo 3 than the current retail version.  If this was an expansion pack for Halo 3 I would expect that it would contain a campaign mode.  The campaign option my not yet be ready so that could explain it's absence.  I can't imagine a whole new multiplayer game since it would fracture the Halo 3 community.  

What I do think this could possibly be is a Halo 3 MMO (massively multiplayer online game).  Think about a Halo 3 MMO FPS in the model of a World of Warcraft style game where you have loot drops and quests.  The Halo brand is probably the only brand that has the clout to take on World of Warcraft.  This might be a shot in the dark but who knows.  You have party support in the Xbox 360 and this would explain why there is no campaign int he game. Eventually this game will go public and we will all find out what Haol 3 Mythic will become.



Source:
http://www.addictgaming.com/