Time to chime in here on the Adeptus Arbites conversation, simply because I was there in the right spot when the conversation came up with the GW team during the NOVA Open event.
This has been making the rounds... out of White Dwarf.
In this months White Dwarf, in the Temporal Distort feature on page 132
".. the Adeptus Arbites haven't been seen around much lately, but with the likes of Genestealer Cults on the rise again, who knows when they might be back?"
This is what was said at Nova....... Necromunda Underhive will not have the Adeptus Arbites in it. Underhive is below the Arbites reach in tight fitting corridors underneath what we know from before in Necromunda. However..... they do plan to expand the game outwards, which is where the Adeptus Arbites were mentioned. There may be some other connections as well with missions etc.
Now this is all taken from memory, because it all happened faster than you can take notes and I was writting other things down that were being said.
Did they rule out a new 40k Faction? No. In fact I believe one is in the works (unverified atm, but Im working on it). So is it possible that the Arbites will come to 40k, yes, but from what I've heard it seems rather unlikely anytime soon, and all the current chatter seems most likely for Necromunda.
The Forgeworld guy there (really sorry I did not write down his name), really went into the flavor of the new Necromunda Underhive game, explained why it was in such tight corridors, and mentioned expansions outwards out of the tunnels.
So expect the Adeptus Arbites to be coming with Necromunda, possibly even earlier than expected from the big Nova reveals, with the mention of them in White Dwarf. Plus they are very popular. Hold on though for going beyond that.
If someone else that was there has a different take on what was said or can confirm additional details, please join in and let me or us know, after all I was exhausted from flying all day to get there, and rushed in to the conference to get what I could.
So Salt on Adeptus Arbites rumors atm, and on a new 40k faction.