What is the most interesting seed, thread, plot, or minor thing you've found mentioned in the books, or single book, in the line?

For example in DH2 (as I'm posting this in all the game threads), I found the Skydock 4/4 heresy. I found it pretty interesting and there's a few ties across a couple of the books as well, but it's by no means prominent in the line.

Basically looking for things to look into in the lines to have a good idea of one game I'd like to run from each core.

I like how, in addition to being connected as three books, the end of the Warpstorm Trilogy can, and does, if you did, link up to both Lure of the Expanse, and Edge of the Abyss. One of my big gripes with some game lines is "stand-alone" syndrome, in some of their books, like they are so terrified that they haven't tricked you into buying all their stuff that they never reference anything other than the book in question, and the core book, if it is a different book. Some character stat blocks, in things like d20, can REALLY suffer from it, when they give the character, but not one of the many things that character might be famous for, and so they lack, or you need to adjust for it, even as they copy-paste plenty of other things, but often, near the end of a line, they finally start saying "if you have access to _______ book, this is a thing to consider!", and Fallen Suns does that, which I very much like. it's a little thing, certainly, but that's me. Makes it feel more like an interconnected, ongoing story, throughout the Expanse, if you run those modules, which I like.