Hi everyone!
I cant help it but Im in a hardcore speculating mood right now
I’ve been thinking hard about the implications of everybody in Halo 4, and about what the realities of the “ancient threat” might be…
Please reply with your thoughts!
SPOILERS
To try and clarify I’ll post links throughout, so please bear with me
Firstly, the Precursors were always said to be able to travel between galaxies.
(Important: Precursor knowledge, so far, on the Halopedia website: NameBright - Coming Soon)
At first the Precursors deemed the Forerunners to be their successors, and were going to pass the Mantle on to the Forerunners.
(Check the previous link for more on this.)
But long after the Precursors seeded life across the galaxy, they deemed the forerunners unworthy of this task. By then, the well-established Forerunners rose up against this and brutally destroyed the Precursors, despite their technology. So, in a last effort the Precursors created the Flood to kill them. And what direction did the flood arrive from? The MAGELLANIC CLOUD GALAXY!! 
(check out this page and scroll down to Origins: NameBright - Coming Soon)
So I think the situation of the Precursors is much the same as what the Forerunners did when faced with the flood; they were doomed, so they abandoned the galaxy, and killed everything they left behind, in order to exterminate their enemies… Except the Precursors had the ability to go elsewhere and to other galaxies.
Since 343 confirmed Forerunners in-game, and the Flood arrived from far away i think theyre both still ALIVE!!! And maybe the Precursors won’t even show up in Halo 4, but theyre going to have to appear sometime! The Halo series is finally going to reveal its true origins either way 
Lastly just to uber-speculate on things, perhaps as things play out the Forerunners will be teaming up with you even against the precursors? Lets not forget there will be Halo 5 & 6 as well… :]
Thoughts?