Watts will play "a charismatic socialite hiding a dark secret", which is about as much information that we have of her involvement in the new series based on the books of George R. R. Martin.
However, stellar producer and writer Jane Goldman is helming the series, which is to be entitled 'The Long Night', so the show should be in very capable hands. Goldman's previous projects include 'Kick-Ass', 'X-Men: Days of Future Past', and 'Kingsman', and she's also writing the script for Disney's live action take on 'The Little Mermaid'.
Page Six reports that the prequel series will be "Taking place thousands of years before the events of 'Game of Thrones'. Goldman’s project chronicles the world’s descent from the golden Age of Heroes into its darkest hour. And only one thing is for sure: From the horrifying secrets of Westeros’ history to the true origin of the white walkers, the mysteries of the East to the Starks of legend – it’s not the story we think we know."
Going from this, we can expect something completely different from what we've already seen - which is a very good thing. However, Watts' prequel series is apparently one of five 'Game of Thrones' projects currently in development at HBO, so whether 'The Long Night' will ever see the light of day is anyone's guess.
The final seventh season of 'Game of Thrones' hits TV screens in 2019.