I voted Skellige, Flotsam, and Kaer Morhen.
I found that TW3 didn't do justice to Skellige. I would love to revisit it again in both main and secondary storytelling. Plus you can never get bored from just wandering around with Roach along with that epic free roam music in the background. I just hated it when suddenly a nosy monster would go "Aaaarrgghh" out of nowhere and breaks the rhythm.
Flotsam will always remain special to me. It has many reflective parts that shows multiple dichotomy yet applied on a manageable sized map that helps keep things under control without getting lost amidst story progression. The design of Flotsam looked superb to me as I genuinely felt Witcher fantasy vibes. I would like to revisit it as well but maybe in secondary story-lines. There are possible loss-ends that we could further explore, such as the case in certain endings with Malena, that I think would be fitting on the scale of secondary plots. Similar to Skellige, I always had moments simply walking around while the beautiful music of Flotsam is playing in the background, but without some nosy monsters that break the rhythm. What can I say, I am big fan of free roaming while cool music is playing.
As to Kaer Korhen, I think none of the episodes done justice to it, or at least the concept of Kaer Morhen. I would like to see it or something akin to it that really serves as a stronghold or manor, not just to be a pass by location. I can think of two examples at the moment which are Villa Auditore in Assassin's Creed II and settlements in Fallout 4. Ezio's stronghold felt like a continous exprience throught the game, regardless of main or secondary quests, especailly due to its level of interactivity that extends not just in terms of accessing the location, but on its functional level as well that prolongs even after completing the story. Admittedly, and naturally, that level of interactivity winds down as we progress, but it still remains there and the achieved progress bears fruit. Similarly, and on a much greater scale, settlements in Fallout 4 goes even way further on the functional level of interactivity sort of indefinitly. Though, I found the building mode was quite constraining. I am eager to see Kaer Morhen or its counterpart to recieve similar treatment that flows through the entire game whether there are main/secondary tasks or none at all.
Overall, I look forward to see something new and fresh as well, but having that level of novelty does not necessarly mean that everything has to be remade from ground-up. There are endless possiblities by just revisiting previous locations and implement further progression elements to them in a way that makes them quite innovative. This is of course not to say that it may seem as an easy task to recalibrate prior locations, on the contrary. In fact, and in many cases creating something from scratch is much easier than revisiting past assets, but the point is that not everything has to be completely built from the begining to provide a sense of innovation. No need to reinvent the wheel.