I think there are some great ideas here…All I would add is that it FFG seem to dislike supplements of a single classic RPG “type”. By this I mean they seem to prefer a crunch/fluff split within every book. Even the bestiary books contain a lot of background material and plot hooks, while the ostensible “sourcebooks” of the various lines (Achillus Assault, Disciples of the Dark Gods etc) contain a lot of “crunch” material of immediate use to GMs.
So it seems to me that some of these book ideas could be split into broad classes that could contain a mix of the material set out above:-
An Exploration/Colony sourcebook, containing a detailed background history of exploration and colonisation attempts within the Koronus Expanse, equipment, vehicles, ship components, space stations, and starships for exploration. Perhaps some hard and fast colony creation rules. Exploration and colonial endeavours.
A Dynasty sourcebook, detailing the full history of the major Rogue Trader dynasties of the Expanse – not so much the individual Rogue Traders and their ships, but their holdings, intrigues, enemies, assets and major ongoing endeavours. Full Dynasty creation rules, enabling players to set out full details of their own dynasty’s assets and history. Equipment (including ships and components) with a decidedly luxurious emphasis – the “bling” equipment beloved by a certain type of Rogue Trader. Rules for Star Yachts – pleasure vessels of the richest dynasties. Guidance over Roguie Trader Politics and their relationship with the Imperium in general and the Calixis Sector in particular.
As for a xenos starship book, I dunno…BFK contained rules for all of the ships that are likely to be encountered within the Koronus Expanse. Perhaps the way around this would be to do a complete “Crossover” book, as Alox suggests, which contains rules for those Xenos threats (and other threats) found in other parts of the FFG galaxy. So Tyranids, Necrons and Chaos forces from the Jericho Reach combined with Ork, Seperatist and Dark Eldar from the Spinward front. It could also cover Space Marine ships, as these are common in the Jericho Reach, and far from unknown in the Calixis Sector (home of the Storm Wardens.) This could be a highly crunchy book containing a basic bestiary of xenos from these various races, together with some lightly “fluffy” history of Rogue Traders in other regions and guidance on how to run a Rogue Trader game in multiple sectors.
All of the above are variations on other people’s ideas; I’m really just brainstorming with ideas set out on this thread already.
For my own part, I would really like to see a balls-out large sourcebook from FFG. I used to love the background-heavy regional city books of old WoD, and I’d love to see one set in the Koromus Expanse. Even GW used to get in on the act with this sort of book, with Middenheim:City of the White Wolf being the model for my favourite type of RPG supplement.
Specifically, I’d like to see a “Mareinburg: Sold Down the River” style sourcebook for Footfall. Imagine a detailed, 256 page supplement covering the history, culture, geography and personalities of Footfall. I’d buy that in a heartbeat.