The House of Kings is the only house we have yet to fully pursue. As of now (October 2015) they are only located near the Array in the Cosmodrome (and on their respective story missions). This leaves the Kell of Kings as the most dangerous Kell at the moment and possibly a basis for future dlc. Let me remind you the Kell of Kings is who previously attempted to unite all the Fallen houses and led the assault on the Last City aka, the Battle of Twilight Gap.