After reading Halo Escalation, this is what I believe the plot of Halo 5 will be centered around:
In the latest Escalation Comic (#16), Halsey and Jul 'Mdama escape with the Janus key, and use it to discover the location of the Absolute Record (the place which lists the whereabouts of all of Forerunner technology).
The Storm Covenant, I presume, (perhaps) alongside the New Colonial Alliance (Insurrectionist group Initiation & Escalation) are the ones attacking outer colony worlds.
The Storm Covenant and New Colonial Alliance being allied up is a bit far-stretched, but there were Elite mercenaries working with the NCA in Escalation, so it’s possible.
Anyway, as I was saying, the attacks on the outer colony worlds (mentioned in the description for Halo 5: G) are surely done by the Storm Covenant and 'Jul Mdama looking to stake their claim for power.
Not only will they have every piece of Forerunner technology at their disposal (perhaps another reason why they’re attacking colony worlds- to own Forerunner structures perhaps hidden on these worlds), but they’ve also been trying to create other super-weapons.
E.g., Halo Nightfall- they hire miners and middlemen to mine the substance off Alpha Halo which selectively kills humans,
& Escalation- the earlier story with the Jackals- they’re testing a biological weapon which could be used on humans.
Of course, all of that doesn’t particularly explain what 117 is up to. What does the Master Chief have (‘the seeds of our future are sown in his past’) that could influence the colony worlds being attacked?
Well, perhaps, if he does in fact have the IsoDidact’s geas imprinted within him (as many people theories), maybe he is somehow capable of controlling Forerunner technology- or, at least, disabling the access to all the technology so it can’t be used by the Storm Covenant.
Or, maybe it’s related to what the Librarian gave him- the immunity to the composer?
Or, having the IsoDidact’s imprint, maybe he is the only one capable of releasing Mendicant Bias from the Ark, and Bias is able to stop the Forerunner technologies- being used by 'Mdama- causing havoc on the outer colony worlds (remember, he wants to atone for his sins and is likely to assist the Chief.
Anyway, I’m rambling. Food for thought.