In my opinion, the most fascinating parts of 4e's creation myth, and the organization of the metaphysical nature of existence, is that of the contributions of the Primordials and the Elder Spirits. The Primal power source is my personal favorite. I think the Spirit Realm as the haven for the Celestial Bureaucracy and the Elder Spirits as the various Ministers would fit thematically and serve the setting well. The Primordial powers would be the terrestrial, elemental (hence demonic) forces that govern the physical form of the earth and the underworld (Yomi) with actual geographical continuity with the prime world. These would be corrupted, fallen Ministers who have designs on either regaining their lost Ministries or outright undoing the Celestial Bureaucracy.
Setting up the yin and yang of the elemental powers (unbridled, raw, self-serving power) + Yomi (as the conception of hades, yet as a true terrestrial underworld but not the sole land of the dead) and the Elder Spirits (constrained power, humility, everything in its right place) + the Spirit Realm (as the conception of celestial harmony) as the primary governing forces of the metaphysical would provide (a) setting authenticity, (b) a relationship with the 4e creation myth, (c) inherent strife within the celestial bureaucracy and terrestrial existence that would prompt fairly obvious PC bonds/allegiances. There could be corruptible Elder Spirits on a dangerous path and Primordial Powers legitimately seeking redemption as Ministries change hands with entities ascending and descending.
I think this arrangement would put Epic Tier OA play in the neighborhood of Diablo with very focused themes for the PCs to engage with (as I find the Primal Powers book and the various Primordials' stories to be very clear, noncomplex, and naturally at tension with one another). Where the Celestial Emperor fits into all of this, I'm not sure. Perhaps he would be the only legitimate Divine (power source) entity and have formal veto power over ascension to and removal from celestial office (and attendant fall). Sort of like the distant AO in FR but slightly more involved.