I think I just filled in the hole we have regarding the other marines. [spoiler]Leading up to the end of the game, we see ghosts of other marines, using high powered armour. I believe that these marines are the marines that fought alongside Doomguy after DOOM 64, and the one that were sealed in the tombs. I also believe that they may have died in the tombs, due to them not having the Hell-Forged armour Doomguy has, but their souls still live on, to strong to become lost souls. Hence why they're ghosts.[/spoiler] I think that works.