Well, the obvious answers are Dark Eldar and Necrons. Next would be some combo Codicii. For example, 2nd Ed promised a Forces of the Imperium. A Codex with entries for Adeptus Mechanicus, Adeptus Arbites, and maybe a Rogue Trader force would be interesting. Alone, I doubt any of those entries would be able to sustain their own Codex but bound together it might sell. The Arbites could have a rule allowing them to be included in Guard or Inquisitor forces or used as a stand-alone force. The Rogue Trader could include the option of having a limited number of selections from other races as well as some choices from other forces. For example, take a single Eldar Pathfinder or Ranger as an Elite choice or a squad of Kroot or Ioxatl mercenaries. The HQ would be the Rogue Trader with a retinue similar to an Inquisitor. The Mechanicus has the best chance of being fleshed out into a full-fledged army but seeing the other units available would be nice for some special scenarios, fluff, and interesting theme armies. If they ever went back to Chapter Approved style armylists, they could see the light of day.
The Inquisition being rolled into one Codex makes sense and allows all three of the branches to finally be represented. Adding a couple of other Assassin Temples into the mix could be fun but not necessary. The Venenum Temple, for instance, could have a chance to wound a character before the first turn and infiltrate onto the board but otherwise be a mostly close combat model with a type of poisoned attack. Creating additional units for the three branches would be nice and with the space not wasted on repeating the Assassin, Stormtrooper, Inducted Units, and Inquisitor HQ/Elite entries would give additional pages for the new units. Still, it would either cut down on fluff pages and/or color plates or be a pretty big Codex.
I would also like to see a Chaos Marine Codex for something besides the Red Corsairs. The current Codex is OK but I would like to see the Traitor Legions return to the fore. Several could be included in each Codex and make it a series of 2-3 Codicii. I don't think,no matter how much we wish, that each power will get its own Codex. A return to something like the Realms of Chaos books would be more likely but I'm not holding my breath.
Also, I think it would be a bad idea to release the Blood Angels/Dark Angels and Black Templars in one book. As long as the current Codex Space Marines is in publication that kind of cross-referencing to the main Marine Codex would work fine but as soon as it was replaced or changed that multi-Codex would be left pretty much high and dry. Instead I think they should each get a Codex like the soon to be released Space Wolves.
That pretty much leaves just the Tau. The 'Nids seem to be coming soon according to the rumor mills and that is fine. So, I guess the Codex I want is ALL of them. Muhahahaha. All your Codex are belong to me. Even if I don't play an army, I have always bought all of the Codex books. I enjoy the fluff and seeing what to expect out each army. Plus, I try to help the folks in my group come up with new ideas and having the Codex on hand makes that easier. On top of that, since we game at my house it is nice to have a copy of the different Codicii on hand in case there is a question or a forgotten Codex.
Remember, Jervis, ALL of them. Give them to us, our Precioussss-es. All of them, yes. Soon.