I suspect at this point most of the content for whatever's planned that's involving the Night Lords is finalized, but I figured I'd bring it up anyway just in case, or maybe Neocore's already beaten me to it, which would be awesome.
Anyway, in the awesome Night Lords trilogy written by Aaron Demski-Bowden, he introduces us to pieces of Nostraman, the language of the Night Lords' lost homeworld. Over time, he added on more and more, to the point where you can almost have basic conversations with the language. There's a guide for learning the basics of it here: https://forum.eternalcrusade.com/threads/learn-the-tongue-of-the-sunless-world.53435/
If possible, it would be amazing if whatever Night Lords opponents we face have some phrases spoken in Nostraman. I'm imagining some kind of story sequence that starts with you just wandering an Imperial facility full of flayed corpses, then suddenly the lights cut out, you hear a demonic voice, "Solruthis veh za jass." And now you're fighting for your life against a kill-team of Night Lords. In the dark. Or just random phrases called out during more normal fights. Whatever you guys come up with.