Lots of extra fluff taken from pretty much anything I've ever read. If it was interesting, I'd include it. I've also experimented with many, many variants of the Imperium and tech over the years. This included running the "same campaign" over multiple times from the start point and explaining to the players that we were simulating different timelines.
They eventually figured out that these different timelines were connected via a "War of the Gods" metaplot involving Grandfather and other similar beings. Quite fun for everyone involved.
Currently I'm designing my own setting, but again stealing liberally from anything that I find interesting, along with incorporating all the bits my players really liked from the previous campaigns.
I've been pretty lukewarm about running games in the Imperium, but I'm getting a lot more interested after reading The Great Rift and the Bowman Arm. Those areas just electrify my brain with ideas.
I am still chugging away on my Piper Out Rim Sector, as far from the official Universe as I can get. I am using sort of a mix of Classic and Cepheus Engine rules, but modifying the Cepheus Engine rules as needed. As stars are a bit more spread out, J-3 ships are quite popular, along with slower Piper hyperdrive ships.
I do need to work up some magic rules eventually for the really far-out planets, plus the odd inter-universe portal.