Parmandur
Book-Friend, he/him
Planescape had a lot of stuff written in character. I imagine they could find a narrator for that to do a guide to the planes. Or at least Sigil and the Outer Planes, branching them off from the rest for a tighter focus and more detail. Include some details on the gods and religion.
It'd be a good place for other factions and maybe some more factor/reputation ideas. Some planar monsters of course. That'd be the place for extra divine rules: benefits for non-clerical worshipers of gods, rules for becoming demigods/powers, etc.
"In character" for Planescape might be a bit much; I predict Eliminster, Planewalking country Wizard as a good fit.
The Big Book of Crunch, judging by the test articles, appears to be a DMG2/PHB2, perhaps with a twist?
The DM/Player/MM trifecta seen in Volos seems sound: Crawford actually talked about that a bot in the latest podcast. Along those lines, I could see:
Faiths & Avatars style book, giving DM lore (Asgardian Mythos, Olympian Mythos, Pharonic mythos, etc.), player content ( [MENTION=697]mearls[/MENTION] has mused on Twitter about deity specific Cleric domains, Domainof Ares instead of War; races and such) and monster stat blocks and sundries.
Volos Guide to Dragons, going in depth with all the major Dragons, including Gem Dragons, as they do in Volos (complex tables for Dragon personalities by type, please!), player resources like race info, subclasses, items and spells, and monster stats and sundry.
Something combining the Expert or Companion Sets with Wildnerness Survival Guide and Dungeoneers Survival Guide: hex crawl rides again! Maybe epic combat rules included, monsters appropos to army combat, etc.
A world builders guidebook, with tables galore, new player options and monsters (always, MAOR MONSTERS), to facilitate home brew or plane hoping/spelljamming play.
Anyhow, some random thoughts.