And now we have word that there is a new item under expansion05 on the PTR client. For clarity, as far as we know, we are currently on expansion number four. Again it's worth taking all of this with a very large pinch of salt, but while the file itself is simply a bit of grey stuff, the name of the folder it's in is "Iron Horde", and its name is nex_envmap01t.
This is where we really start to speculate. This could be absolutely nothing. It could be a piece of iron that is in a horde area or on a horde outfit, or it could be the name of the next expansion. It could be the Horde's texture of an iron thing, or it could be a new offshoot of the "true Horde" that are coming to terrorize Azeroth in 6.0. Or it could be a red herring put there by the devs to confuse us.
Obviously, this is rampant, unapologetic speculation. It should not be taken as fact, and nor should anything else that's datamined, but it's fun to think about these sorts of things, right?