Apparently the rumor mill has now churned out some tidbits that some Necron and IG releases are next, then Black Templars. We can't really speculate beyond that. GW seems to change things on its own whim. I remember hearing about the Dark Eldar a long time ago in a podcast with Jervis and Jes Goodwin sometime ago, around when 5th edition came out and it took them years to release it. Seems like they sit on projects.
Tau Empire players, you have to wait longer. Maybe indefinitely. Maybe Tau will end up like Sisters of Battle. Who can say? I expect a marine codex next and who is left? Black Templars. Because 40k could always use another gothic, fantasy-import, marine army. 3 are just not enough.
In the meantime, play Infinity! There are new releases for the armies every month!