As for Traveller books, I'd like to see a book detailing the cultures of the Imperium, like Alien Modules but for the human regional cultures inside the Imperium. I read the Third Imperium book, but it didn't provide enough information.
I'd like something like this because I have trouble knowing how and why to play an Imperial character (or K'Kree, Hiver, or Zhodani, but I don't bother with them).
Please don't tell me to go the IMTU route because that fragments people's understanding of what is true in the baseline official setting. If I go to play Traveller at my FLGS or if a new player sits down at my table, it's important to have an official baseline as a foundation before making changes.
Anyway, the Solomani have their ideology and their opposition to the Imperium, and they're from Earth so we have a good idea how to play them (any setting appropriate version of an Earth culture). The Vargr have their charisma dynamics and their wolf-related cultural traits, the Aslan have their honor, clan system, gender dynamics, and desire for land, but what do the Vilani and their derivative Imperial cultures have?
The Vilani are the majority population in the Imperium, but we don't know much about them. Do they value honor, truth, and justice, or is corruption a normal part of doing business? Is consideration of others a value in Vilani culture, or is it more about nice guys finish last? In Vilani culture, what is the relationship of the individual to his family, his neighbors, his society, and the state? What can my character expect, and how should he behave, when he arrives at an average culturally-Vilani world? What is interesting about culturally-Vilani people that would motivate me or the players at my table to want to play Vilani characters? What is the Vilani aesthetic? The answers to these questions are what I would like to see in the kind of books I'm suggesting. This is important because what I've noticed in my experience is that people play every type of character besides ordinary "Imperials", and when they do, their characters are "normal contemporary people in space", so to speak. Also, the DGP books are long out of print, and in my opinion no longer relevant. I want books about this written by Mongoose for the Mongoose Traveller setting.
I doubt very much that the influx of Terrans during the Rule of Man or the influx of Sylean "Solomani" during the Consolidation Wars would do much to change the demographics of the 10,000 worlds. Their cultural and technical innovations would most likely have a great impact after the Long Night, but these changes would add to and not eliminate a population's sense of who they are and how they've interacted with each other for at least 1000 years.
I'd like to see books that illuminate the various human cultures of the Imperium so that every one of those cultures provide interesting compelling reasons (this is who I am, this is my way of life, this is how to behave correctly in my society, etc.) to play characters of those cultures. I've noticed what without those reasons, players tend to default to no characterization at all and a lack of interest. It would be great to have interesting plausible conflicts between those regional Imperial cultures, so Imperial characters could have differing viewpoints.
If there's enough interest in discussing this, I'll make a thread for it so it doesn't distract from this thread.