If one of Oryx's sister's sought the 'pinnacle' of sword logic, it is likely that they will come after the PC Guardian in the future.
After KF, the chain works like this:
Guardian>Oryx (who is now dead)>Xivu Arath/Savathûn.
The Guardian is the new pinnacle. Either these sisters intend to take the pinnacle for themselves or they want a warrior of light to be the strongest (for some inexplicable reason).
I don't think Bungie puts this much thought into their story, though. Maybe we're overthinking it.
The lore is pretty good.
Makes me wonder why the in game story is so lacklustre by comparison.
I guess you can do more with simple words than you can with expensive models and voice acting.