So in halo 4 some of the concept art showed jungle and the interwinding of forefunnerstructures with the vegetation but it also showed other life such as this that i snatched from the web.
So your probably thinking, " yeah so, and?" … well the same kind of beings are found in the background in the halo guardians concept art here:
same tentacle floating beings which kinda confirms that the main plot will take place on another forerunner facility like Trevellian for example or the ark as everyone keeps saying.
As that theory does make sense, the distance to travel to such a place is far beyond human capability even with the inherited forerunner ships that were found in Halo onyx.