Thronebreaker was announced in August 2017 as a singleplayer campaign for the Witcher-based card game Gwent, expected at the time to be out by the end of that year but later pushed into 2018. A year down the road, the plan has changed again: CD Projekt said during a financial results conference that the mode has "evolved" into a standalone RPG called Thronebreaker: The Witcher Tales.
It will not be a full-on Witcher-style RPG, as CD Projekt president and CEO Adam Kicinski said that most of the action will be resolved by playing Gwent. But it will offer an estimated 30 hours of play, he said, built around a story "with choices and consequences."
Thronebreaker as it was originally conceived was based on Meve, Queen of Rivia, who knighted Geralt for valor at the Battle for the Bridge on the Yaruga during the second war with Nilfgaard. She's only mentioned in passing in CD Projekt's Witcher trilogy, but she's a formidable figure in the lore and could open the door to some very interesting stories—if she's still a part of the game.
Kicinski said that Thronebreaker will be released as a standalone game, but unfortunately gave no indication as to when that might happen, or what it will cost. I've emailed CD Projekt to ask if Thronebreaker has been delayed further because of the change, and will update if I receive a response.
CD Projekt's Capital Group H1 2018 financial results conference stream can be seen below. The Thronebreaker bit starts at 38 minutes.