So, I'm going to go into a little more depth here (as I didn't want to clutter the Wowhead thread or post) but here's my logic behind my main theory:

Almost every hidden artifact skin has been found, and almost every one of them has been a logical source (Havoc DH and Ret Paladin being outliers. Who the hell thinks of killing a giant bat??!?) From there you have to ask why and how come.

Was the warrior community slacking on looking for this? Have they not been scouring the wilds of Legion and Azeroth looking for it? Why weren't Warriors as voracious to find this thing as Frost and Unholy DK's? Not really! There's enough people looking for it, just nobody had found the answer yet. So my immediate reaction was to go right to the lore and artifact knowledge notes (I'm a Lore nerd! I've been playing a warrior since Vanilla! Surely I of all people can be the one to solve this riddle!). I must have spent five hours pouring over Lore and axes and transmogs and storylines on wowwiki and wowhead to find this thing. I thought of Gorehowl, I thought of the Axe of Cenarius wielded by Brox, I thought of the Arcanite Reaper, Ripper, and the lookalike from Molten Core, and Nightfall crafted by Blacksmiths. There was Thura and all the Saurfangs and Nazgrim and the Amani Trolls. You name it, I've been over that! I even thought I had it when I went through the fliers in BRH and realized it was a map of Gilneas. (It wasn't, spoiler alert...)

Then it dawned on me, I'm overthinking this. When in doubt, take it back to basics. If something hasn't been solved yet, in ALL this time, why not? It's. Gated. By. Time. What in Legion is gated