I imagine I'm in a minority here, in that when picking up a new release I get more from the worldbuilding and the setting information than from the mechanics -- I'm driven more by fluff than crunch, basically -- so other, more hardened players would need to add in ideas for making these fully appealing for everyone...
But:
I'd like more of the empire/location books. I'm anticipating the Third Imperium volume and its fleshing out of Core Sector, as well as the Spinward Extents book (which is basically, unless I'm gravely mistaken, either new material or reworking of really old material). After Third Imperium, I'd love to see similar releases for other nations -- really get into them, so the best use could be made of alien Travellers in their own space, or as background for fish-out-of-water "Imperial or Solomani humans abroad" campaigns. Plus it would be an excuse for interesting and distinctive new vehicles/equipment/etc.
Zhodani Consulate seems obvious (and building on the intriguing first edition Zhodani book would be welcome); I'd kiss a Girug'kagh for the Two Thousand Worlds. But even more than the Two Thousand Worlds, I'd love volumes detailing:
The Hive Federation: Let's look at all the fun clubs with their weird agendas. Lots of potential for colourful, somewhat surreal politics and intrigue of a very different flavour from the usual Imperial nobility/SolSec. Let's also get the Ithklur, Gurvin, Za'tachk and Federation Humans fully fleshed out. (Let's introduce a few of the Hiver's-Friends-and-Relations that never get named, while we're at it).
Julian Protectorate: I've said it before, but I think this would be a great region to set a campaign series in, beyond simply detailing the area. You have multiple nations with different customs and cultures but a loosely unified framework for movement and exchange between them. All sorts of economic, diplomatic, social, political and criminal adventures are possible; you have a unified authority but it's *very* light, and the member nations would give many options for distinct character backgrounds. You have Humans and Vargr from multiple cultures, many joint-species, plus the Hhkar for some alien colour.