Let me preface this off by saying I thought of this at 3 in the morning under the shower. Lets go ahead and get to my theory:

Remember the HoW playlist where Skolas takes control of the Vex technology and tries to wreak havoc by pulling the House of Wolves through time? Skolas was able to harness the Vex tech and try to use it (before our badass guardian captured him.)

Oryx is a smart man, I assume. What if Oryx either:

a.) Captures the Vex and harnesses their technology

OR

b.) Takes the Vex and makes him do his bidding.

Where am I going with this? What if Oryx drops into the Vault of Glass with his army of Taken, and either takes Atheon, or harness his power, and go into the dark corners of time. You know, where Kabr and his fireteam were lost.

What makes it seem like this happen is the description for the Praedyths Revenge, specifically and it will happen again. Sounds like he's coming back, right? What if he's coming back AS TAKEN? Who knows the weaknesses of a guardian, better than a LEGENDARY GUARDIAN?

TL;DR: I think Oryx is gonna go into the Vault of Glass, harness Atheon's power, and take Kabr and his fireteam for you to fight against.