the chief and cortana live. The special ending on legendary isn't that much more. it shows the regular ending after the credits were the chief and cortana are floating in space and the chief enters the stasis tube. ON the legendary ending it goes for a few more seconds and you see them fall into orbit of what seems to be an inhabited planet of some sort. Is that a prelude to a #4? who knows, but if it is count me in i'll be buyin it.
At least its more closer than halo 2. Man was i ticked at that game when i beat it and found i was left hanging and had to wiat foir over a year to see what happens next!.