I would strongly prefer if the endings of Wild Hunt were not destroyed, and the previous games did not have "canon" endings, even if their choices did not matter much in the sequels. Anyway, as the default ending, I suggest:
Ciri witcher if she is the protagonist (kind of hard to avoid then), witcher or dead otherwise, empress could be difficult to import because of the consequences to the world politics, it would also imply Nilfgaard winning and lock out other endings. I do somehow suspect that a Ciri sequel is a possibility, even if I would not like one myself. Although I can also imagine that she would start as an empress (or would be about to become one) in the prologue, but lose the throne for some reason, and the actual story could begin from there.
Geralt alone if he is the protagonist (unlikely, it has been stated many times since about 2013 that his story is over), save import should be possible otherwise. Even if he appears in a Ciri sequel as an NPC, his romances are not important enough to the story that acknowledging them would require more than a few dialogue changes here and there and maybe a Letho-like cameo. If for some reason CDPR do make a new game with Geralt, then locking him into a romance from the beginning would not be a good idea in my opinion. It would be difficult to break it again in a credible way if it is with Yennefer, which also looks like the most "final" ending with his retirement in Toussaint. The ending with Triss seems to be a more interesting starting point, but it could very well just end up being a repeat of TW2. No romance after skipping some years gives the writers the most freedom, and it is also the ending where he remains a witcher and does not retire.
Svanrige rules Skellige, no particular preference here, it is just the "neutral" option so I pick that if there is no good reason to choose otherwise. Anyway, I think that Skellige would not be a location in the next game, with it being a fairly isolated place and the existing titles not recycling locations when not really necessary. Therefore, with no quests taking place in Skellige, the relevance of this choice could be limited to some brief mentions in dialogues or readables, and then implementing a proper save import should not be difficult.
Radovid or Nilfgaard wins, but I do not have anything against Dijkstra either, it just somehow feels less likely. This choice seems to be quite hard to import if the new story does take place in the Northern Realms again. It would also not be easy to contrive an explanation how all the endings ultimately lead to the same outcome (Radovid, Dijkstra and Emhyr all assassinated and someone else wins?), unless a relatively long time like 10-20 years passes between the events of the two games. Ultimately, the best world state is whatever suits the story of the sequel the best, but we do not know enough to tell for sure what that would be.
In general, judging also from how it was done in the existing games/expansions:
- optional content not done: that rule alone leaves only 2 endings (Ciri witcher/dead, Geralt alone, Radovid, Svanrige)
- if an NPC can be dead, then dead
Note that the above are not necessarily based on what my preferred ending is, but rather what I think would be a decent starting point from the writing point of view. The happiest or most popular world state is also often not the one that is the best to start a new game from, for an interesting story, one might actually want something the main characters are not entirely satisfied with.
In the end, I would prefer a new story that is not a continuation of Wild Hunt - all the endings remain equally valid in that case.