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.

The idea that a necron has been thinking of using marine bodies to allow necrons to reincarnate into flesh was a good idea. Also the working of that necron chronomancer from the subekhar dynasty and the implicat5ion he was gbeing thrwarted by inquisitor ramaeus could have been interesting., Inquisitor Ramaeus herself was never really used.

The reference of the Storm Wardens chapter purging Slaught from tunnels under the surface of the (Calyxis Sector) planet Vigil, suffering heavy casualties and leaving them eager for retribution against the Maggot Men. Since my Deathwatch campaign is set in the Calyxis Sector, I plan on having an unexpected run in with the Slaught at some point, and I'll try to goad the Storm Warden player into going 'Captain Ahab' and derailing the mission in the pursuit of vengeance.

Slaught are always fun, I remember when my group ran into Master Nonesuch in House of Dust and Ash. They're still terrified when I mention worms, but getting a leg disintegrated will do that.

I wish they'd done a full slaugth writeup in Dw. Does that count as an undeveloped plot?

what wad that storyline about a heavily damaged storm warden battel barge thst was declared beyond repair, but a recon.kission sent to evaluate it saw evidence it wsd being repaired abd tbst figures wearing debased storm warden armor seemed to be working on it?