So from what I think is going on with 343 is that they are going to send the Master Chief into the Large Magellanic Cloud, known to the Forerunners as Path Kethona, and while he's there he discovers the planet shown in the trailer at E3.
Path Kethona was where the last few battles of the Forerunner-Precursor War took place, and where the Forerunners annihilated the Precursors to the distant memory of time. BUT! Not all of the Precursors were wiped out... They went into a type of stasis in which they became dust that would reconstitute itself and regenerate into living form again. But as fate would have it this was not to be. How ever the Precursors saw this as a means to an end- Punishment against the forerunners for their insolence..... THUS! the flood.
SOOOOooo! Not only is a old unfriendly alien horror probably along for the ride again, but we will encounter more forerunner items and things that would see master chief dead, a even more powerful and original version of the flood, possibly a few precursors(MAYBE), The UNSC hot on the Chief's tail again undoubtly, The Storm(the covenant faction in Halo 4) will probably rear its ugly and fanatical head again. Thel(The Arbiter) will most likely only make a tiny and brief cameo appearance in a cut scene.
The final topping and piece of this epic space cake is Cortana and the seemily titanic quest Chief is going on to get her back.
Honestly this is a WTF move by 343, to much oddities coming and strung together.(If this is the way 343 is building H5 towards)