I have read that Andrzej Sapkowski is writing a new Witcher book as of 2018. Has he ever mentioned a desire to build more universe, much like Tolkien has done with The Lord Of The Rings series? Not necessarily about Witcher's, but maybe a history of his universe or how the multiverse works.
The answer is:
no YES. See edit below.
To quote him from one of the interviews:
The Witcher Saga is closed. If there will be another book in this universe, it will be something along the lines of a prequel.
Sapkowski for a long time was of the opinion that Witcher is a closed topic. I was really surprised that he wrote Season of Storms, but he did, and as you mentioned he actually plans another book, which will be a prequel.
I think he personally is of the opinion that expanding the universe is boring.
I make that statement based on his words about his other series, the Hussite Trilogy, which he found much more to his liking, especially the research part. This is actually the reason his books are coming out so slooooowly - he is very meticulous in his research, and he said he much prefers history to mythology.
I do see him, however, one day announcing a collaboration with other writers set in "The Witcher Universe". He very much wants to cash in on the success of it, too. Just like CD Projekt and Netflix.
I stumbled recently on the interview with Sapkowski with one of the Polish sites, where he said that there will be some more material coming out soon-ish, which will be placed in The Witcher Universe. He is looking for authors for collaboration. He also announced that there will be more Geralt books, but he also clarified they definitely will be either prequels or placed in between previous books. So, there.