Well, I am really not sure about Unreal Engine 4's current capabilities of rendering giant sandbox areas. Yeah, it is a really nice engine to render open areas comparing to other engines but sandbox area rendering is a big challange and even the World of Warcraft's engine is a disaster when it comes to drawing distance.

There's probably a snowball's chance in hell that they'll do this. It won't perform on the broad range of machines they strive to bring it to. It will be far too much work to port everything over to a different engine. They will probably be too afraid to make such a drastic change to how the world looks and feels (remember the backlash when they removed the hunch from orcs?)
The rational part of my brain has given up on ever playing WoW natively, and I don't plan on using Wine for it either (although it works well with it).

That said, this looks gorgeous, and while I was never really into Elwynn, this did bring up some feels.