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I vaguely recall in the lead up hype to 8th ed that it was mentioned that there would be 2 new factions. A lot of us thought this could be the advent of DarkMech or renegade guard or even the Demiurg. But nothing came of it!

Were GW referring to Primaris and Death Guard as the new factions?

I guess so. And is think it was a bit miss leading.

Was it GW that announced a new faction or just rumour mongers ?

There might have been something else that I don't remember, but there was at least this:
https://www.warhammer-community.com/2017/07/05/codexes-your-questions-answered-july-5gw-homepage-post-2/

As well as classic and well-known armies, you can expect to see factions that have never had their own codex before – starting with the Death Guard, available later in the year.

Doesn't really make it sound like a totally new faction is coming though.

I remember them saying '2 new faction' in one of their videos leading up to 8th.

Probably meant Death Guard and Thousand Sons

40K really needs fewer factions and better attention paid to those which exist. I really hope we don't see any large new factions (sounds like Dark Mechanics is happening though).

Dark Mech, maybe sisters of battle are the two biggest releases I can see them doing. Dark Mech is pretty simple - add a few models and give them access to admech stuff and daemon engines - and sisters have been needed for a while.