ruleslawyer said:
(1) A full-of-flavor pages-from-a-thaumaturgist's-diary (or balor's diary!) set of excerpts on the nature and habit of fiends. Subjective, compelling, and unreliable enough to be interesting reading and fill DMs with lots of ideas, but not spell out a fiendish ecology in any sort of quasi-scientific or hard-and-fast manner.
I like this one.
There was an article in Dragon back right after 3.0 came out written from the POV of a Sage of Fiends.
Enchiridion of the Fiend Sage I think it was called. It was the issue that had the Cataboligne Demon (which, IMO, should be included in the book

) and Gingwatzim statted up. Very well done actually.
The issue number escapes me at the moment, but I could dredge through my magazine piles if anyone is interested.
"Option 1: Fiends as Extraplanar Natives
In this cosmology (ecology/whatever), fiends (and celestials) are simply beings native to the appropriate plane, similar to the way in which elementals are native to the Elemental Planes, or mortals to the Material Plane. Their alignments are perhaps shaped by the planes on which they live (or the other way around), but they do not have any special connection to mortals. (and so on)."
"Option 2: Fiends as Deceased Spirits
In this cosmological option, fiends are creatures formed from the spirits of dead mortals when those spirits travel to the appropriate plane upon death. They may or may not have mortal memories... (and so on)"
"Option 3: Fiends are creatures who represent Cosmic Concepts
In this option, fiends are living embodiments of cosmic concepts: greed, rage, light... (and so on)"
These would be great as little optional sidebars. Present the way that the Core D&D cosmology presents fiends with these little optional tidbits here and there. That would be cool.
Other Suggestions:
1) I said this before in the other thread, but if FCI includes any Demonic Lord info, I think it'd be best presented as deity writeups (ala
Complete Divine or what Orcus got in
Libris Mortis). Short flavor with info on Domains and maybe some sample cultists, sample cults, that sort of thing.
2) Personally, I think it would be cool to more thoroughly explore the 'Lichfiend.' It was presented as an option in
Libris Mortis, but I think it might be kind of neat to flesh it out a bit.
3) I'd like to see the book reprint a lot of the old 3.0 demons and give them their 3.5 power boost.