Essen Spiel is going on right now, and a lot of Europe is there (or would like to be). Its has been revealed at the event that 4 Houses/ Factions will join Goliath and Escher in 2018.
Just a reminder... Goliath and Escher will come with the game, and hopefully additional models in November. So this is a future release for Underhive, which does not preclude a Necromunda Expansion which has been mentioned (and bringing in the Arbites).
Quote via La Voz de Horus
https://www.lavozdehorus.com/las-otras-cuatro-grandes-casas-necromunda-saldran-2018/
"The houses Delaque , Cawdor , Orlock and Van Saar will join the Goliath and Escher that already come in the box of Necromunda: Underhive."
Some information on each House.
http://wh40k.lexicanum.com/wiki/House_Delaque
http://wh40k.lexicanum.com/wiki/House_Cawdor
http://wh40k.lexicanum.com/wiki/House_Orlock
http://wh40k.lexicanum.com/wiki/House_Van_Saar
Just for fun, here is some more info for Goliath and Escher
http://wh40k.lexicanum.com/wiki/House_Goliath
http://wh40k.lexicanum.com/wiki/House_Escher