Generally I think that if there was such a book, it would be all celestials in one book, with a bunch of player-centric material, such as prestige classes that give celestial powers, heritage feats, monster classes and the like. In many ways it would be the companion book to BoED.
There's still a couple things from 2e to fill out like Agathinons, Light Angels, Noviere, and Shiere. It could include the Zoveri and recast them as Archons, and really the only celestial paragon that I think never got detailed were Absalom and a few significant Solars. They could do things like introduce more royals of the Court of Stars, and create new angel castes like Lunar, Sidereal, Stellar, Nocturnal, Mahadeva, Grigorim, Cherubim, and Seraphim.
But most likely it's going to be quite player focussed, and probably resemble the 2e product Warriors of Heaven, more than FCI and FCII.