Prepare to see a lot of pairs of Thunderfury in World of Warcraft. Seriously, a lot. You might argue that it’s rare enough to see one, since legendary items aren’t transmoggable, but it turns out that the developers have delivered something expected for Shadowlands even earlier and legendary transmog has arrived for patch 8.3 on the test server. That means that your legendary cape from Mists of Pandaria, your legendary armor from Legion, and the aforementioned Thunderfury are all available for dress-up if you want them!
This does come with some restrictions, though. If an item has class restrictions inherent to it, only those classes can transmog the item even if it might seem otherwise doable (so, for example, a Mage cannot transmog Illidan’s warglaives despite them being swords). Those that can only be obtained by certain classes without restrictions in the description, though, can be used freely, so you can have your Warrior dual-wielding Dragonwrath if you want. And, of course, some transmogs (like some legendaries) can no longer be obtained. Still, though… buckets of Thunderfury sets. Just wait for it.