I am not quite sure that I understand the idea of these characters earned their rest, planning the Witcher as a trilogy, etc... I mean these statements really blurs the vision of creativity.
I am going to share some thoughts of mine that at a first glance might totally not make sense with this thread, so please bear with me.
Let's say someone is moving to a new home, and that someone is planning to make his new bedroom as a small one, but the idea here is not just the size of the room, rather, it's the contents that he will fill his new room with. So if he is about to move many contents that the planned size of the new room is simply not enough for these contents, then he will need to either expand the size of the room or dispose some of the contents. Of course he can move the extra contents to another room, that is filled with different types of contents, but that will make the already existing contents in that different room not fitting with the contents that he could place in his bedroom, which will make the overall look of the room unappealing.
This also works the other way around, moving small amount of contents into a large room will make the size of that room as a waste because that extra size could've been invested in something else.
So saying that The Witcher is pictured as a trilogy or that its characters had their enough share of the game is not really the case here. It's the amount of contents that can be filled into the game. And, in my opinion, a trilogy is a small sized room compared to the heaps of potential contents in Geralt's story setting.
The small framework of a trilogy even caused some contents to be not fitting together. For example, naming W3 as the Wild Hunt gave me the feeling that I about to face a series of epic encounters with the Wild Hunt. Finally, after all these years. But after finishing the game, I felt that I entered a room with mixed contents that do not fit together. The Wild Hunt, Ciri, Yennefer, Avallac'h, the White frost. I mean how these contents fit together? So the way I see it is that the limited size of a trilogy made the creators to look for another room that's already filled with other, and different, contents in the Witcher universe and try to fit it with the game's setting. And this is of course due to the small framework of a trilogy.
And this is also happen in other games and movies where the creators introduce new characters or events and make the game/movie circulate around them, and may even name the game/movie on these characters/events like Arkham Knight or War for the Planet of the Apes, for no apparent reason because they simply do not fit with the original setting.
That is why the MCU is that much successful because the contents were so vast that cannot be limited in the framework of trilogy, tetralogy, pentalogy, etc... And instead went into phases that provided a lot more room for further character development. So it's not the case anymore to limit certain characters with limited releases, rather, it's to trying to fit the contents together to further develop the characters and events until they reach their full potential. That's why Civil War can be considered as Iron Man 4 because the creators did not limit themselves to the episodic nature of works and relied on the contextual aspects of the MCU events.
Maybe Witcher should follow a similar phase system in order to provide more room for further development of characters and events and achieve different gaming styles. Of course there are financial, time, and technical limitations that can force the creators to leave something out in exchange for something else.
But, personally, I see that leaving Geralt's setting for good will be a significant waste for the Witcher's universe. The most intriguing aspect for me is the consequences of defeating the last crone in W3. The idea of fighting beside the same monsters who we thought were dangerous and must be eliminated is a strong turning point. One can start to realize this in Something Ends Something Begins as Geralt witnessed the bad condition of Berem, the dying she-wolf, and the dead cubs which are all a result of humans' monstrosity. We never saw the other side of the story, of the "monsters". When Weavess told Geralt that these creatures are waiting to avenge their brethren who were killed just for money it reflects that these creatures are in agony for losing their families who were just trying to survive.
There other highly potential aspects that can be implemented in a sequel, but I guess the post now went extra long that I might need another "room" for