We now have three hardcore realms (Defias Pillager, Doomhowl, Skull Rock), each with 1 layer and a dwindling player base. The nature of hardcore does not do well with so many servers.
Most people expect that a cluster similar to era is on the horizon, all of the servers have the same ruleset now, so any input from blizzard on the timeline or feasibility of a cluster would be appreciated.