Honestly I can't see why CDPR would not want to continue the Witcher saga with another DLC or a new game. I realize they focused on different projects right now but there is no hurry imo, since the gameplay is superior to many other games and it won't be outdated for a loooooong time. I read the whole saga and played all 3 games and TW3 DLC's, and finished TW3 3 times on death march collecting all of the trophies (achievements) in the process, and even I am longing for even more content.
SPOILERS BELOW
First of all there are lots of things on the base game that aren't concluded, After the player chooses the faith of Radovid (and by extension, Roche and Dijkstra), the faith of Novigrad is pretty much up in the air. I decided to take part in the assassination plot of Radovid and decided against Dijkstra at the end of the quest, so what will happen to Temeria (under Roche) and a leaderless Novigrad? What will happen to the lodge? Where are the other members? For example I was very sad that one of the DLC's didn't include Fringilla Vigo more, only on the last quest of the main game. She was an important part of the last book and she didn't even feature in Touissant.
Skellige was on the brink of a civil war after one of the En Craite's ascended to the throne, I killed one of the Jarls, that just ended the whole story? Seems like an important storyline that did not really end.
On less important things, what was the thing Keira needed help from Lambert, what happened to them? Maybe something bad has happened and Geralt needs to help them. Oxenfort was greatly underutilized as well imo, there could be lots more story lines involving the professors escaped from Radovid's wrath, maybe Geralt should reinstate the University on one DLC.
On the DLC's, we killed an Ofieri prince on HoS, I was really excited when I thought they were taking us to Ofieri but we escaped, maybe a new DLC would take us there. I mean we still killed their prince and nothing happened. On BaW, vampires were hunting Regis at the end, what happened to him? How is the rule in Toussiant after the events of BaW, I got the good ending but still. The sister should have lots of enemies after what she had done.
Lastly the game didn't even go near many of the lands in the Saga as a whole, Kovir, Lyria, Razair, Bronkilon Forest and its surroundings and Eastern kingdoms are almost wholly untouched. There are lots of opportunities for new quests, and they would be basically cash-cows for the studio and would not even come close to over-saturating the franchise since the world of Witcher is so vast.
I hope CDPR will return to the Witcher saga, a new Gwent game is good and all but it wont satisfy the desire for more witcher.