I'd go for a Celestial Codex series, certainly.
Though, I'd also be satisfied if WotC at least put out a single Planar Codex covering all non-fiendish outsiders/outer-planes of importance. I'd rather much prefer a series of them though, but at least one such book would be nice.
Preferably at least a single Celestial Codex, an Elemental Codex (of all the noteworthy Inner Planes inhabitants though), and a Planar Codex (of all the main inhabitants for the neutral-oriented Outer Planes, and the Transitive Planes, i.e. Slaadi, Rilmani, Inevitables, Modrons, Formians, Mercanes, Githyanki, Githzerai, etc., perhaps even those introduced in the Fiend Folio, such as Ethergaunts, Shadar-Kai, Nerra, and such; and maybe, just maybe, if the stars are properly aligned....horrors from the Far Realms of Insanity?).
At most I'd imagine a trio of Celestial Codices, and a trio of Planar Codices (one for elemental planes, one for inner planes and transitive planes combined (since the transitives themselves have relatively little life), and one for the non-fiendish/non-celestial outer planes; probably released in reverse order, actually, since more folks would be interested in slaadi and inevitables and mercanes and gith-folk).