At the World of Warcraft Questions and Answers conference dedicated to questions from World of Warcraft players mainly about Shadowlands, the developers answered a very interesting question about the future of transmogrification:
Will legendary objects ever be transmogrifiable?
The developers' answer was as short as the question: it's a yes! Before, legendary objects were those cool objects that everyone idolized and refused to see worn by anyone, but it is no longer a restriction that has a purpose nowadays. It will therefore soon be available to transmogrify any legendary object, including the Thunder Blade, the blessed sword of the Seeker!