With all of the upcoming CD Projekt Red games, I wouldn't think to expect any sort of multiplayer mobile title in The Witcher series akin to Horizon Steel Frontiers – but that's apparently what we might get someday, if recent rumors ring true.
The supposed leaks circulating on the web stem from a new MP1st report that claims an inside source has confirmed a free-to-play, co-op action Witcher RPG is currently in development.
This could apparently be the already-known (albeit mysterious) Project Sirius, but MP1st notes the rumored game is planned for only mobile and PC platforms right now – "with no mention" of console launches.