I really want an 'endgame' book.

Rules for marshalling resources and launching a Black Crusade, or becoming a Daemon Prince.

I'd like rules for 'followers' (different from minions in that you have armies of followers) somewhere.  

Tome of Poo
Tome of Lasciviously Cavorting One-Boobies

I'm totally calling this. I bet Slaanesh is last. Appropriate, if you think about it.

I would realy like to see a "Mysteries of the Vortex Revealed" book. Something like The Jericho Reach and the Outer Reach combined, but for the Keening Maelstrom. Screaming Vortex…so blaise'. At least it wasn't named Tantrum Twister.

The idea a buddy and I came up with was this:

Nurgle Book:  Deathguard Plaguemarine and Iron Warriors (possibly a tech-marine like guy), and maybe rules for the Obliterator virus as thats quite an Iron Warriors thing.

Slaanesh Book:  Emperor's Children Noise Marine and Red Corsairs, and then RT-ish ship rules (seeing as though Red Corsairs are very pirate-y).

Chaos Undivided Book:  Black Legion and Word Bearers, then rules for Daemon Princes or some such.

Then a book for launching a black crusade.

Probably won't be Red Corsairs, since in the timeline that FFG uses they're still the Astral Claws and still (technically) loyal.