The Adeptus' Handbook
Simply, chapters covering the various involvements, characters, centres, institutes, opportunities and such across the Calixis sector corresponding to the various Adeptus.
A chapter outlining the variety of the Adeptus Terra across the sector; where do the Tithes go, who collects where, which ones go between planets, which Sector and Subsector worlds are crucial, which are neglected. Where are there Temples of the Priesthood of Terra, who's a senior figure? Where do the Auditors live? Any warehouse worlds?
As N0-1 has been pressing, the Adeptus Mechanicus is a rich source of detail and one I suspect which could easily and well contain enough to fit into a whole book on its own. Still, that'd be a bit of an exclusive book (or rather require a fair bit of work to not make it exclusive). Tons of opportunities I doubt I even need to list.
Then there's the Adeptus Ministorum, Adeptus Arbites, Adeptus Astartes Adeptus Astra Telepathica and so forth. Tons of details within the Calixis sector to make it seem more like a proper... place , but at present, we don't have all of the links, all of the interactions...any of it. I've been working a little bit on Port Wander for the RT game, and I found this to be a crucial exponent; figuring out the 'Imperial mechanics' of the sector (and its relation to the segmentum) not only presented me with a wealth of 'stepping stone' opportunities for my RT players, in even doing a little bit of mental-exploration I found a brick-ton of stuff available to me for DH opportunities. Almost every 'stepping stone' I create could be a massive point of interaction for DH investigators, and this is only one space station! (And I've barely even begun to explore the 'non-Adeptus' bits of the place!)
Anyway a nice more-solid look at 'the Imperium of Calixis itself' is what I'd like to see. Not 'stuff in Calixis' but how it all fits together. The bigger picture and all the hooks that dangle from it, dragging all the 'little people' along with it.