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.