Chikout - DakkaDakka
GW seems to be in a phase of bringing what exists in AoS up to date. Pete Foley said as much in the recent podcast.
In terms of how they are doing competitively the three existing books that most need an update are Ironjawz, Beastclaw raiders and Kharadron Overlords.
At lvo GW said battletomes were still to come for each grand alliance. The Skaven and FEC books were already announced.
So it seems pretty clear that we will get at least one more book for each grand alliance this year.
They just announced khorne and at the start of the year they said there will be more Slaanesh models this year. There are also strong rumours of a Darkoath/slaves to Darkness book this year so that's 3 books for chaos alone.
The rumours about a slyvaneth and KO book alongside their underworlds warbands came from a different person than the debunked stuff.
It was apparently hinted at the lvo event that Sylvaneth are getting a book soon so this rumour could well be true but ultimately nobody knows for sure.
GW are attending Gama again this year from 11th to 15th of March. They usually show off some stuff there so we may hear more then.