As I've heard, while the two expansions are within the time frame of Wild Hunt, they never stated when, i.e. Before climax or after Ciri enters the tower and before the epilogue.
This is their exact wording about the expansions
"they’re within the story of the Witcher: Wild Hunt, running within the time frame, rather than after the ending"
After Ciri enters the tower = Wild Hunt is already 'tamed', unless they specificly mentioned before White Frost.
Wild Hunt refers to Eredin and his army, so gameplay wise, it could be done anytime before confronting Wild Hunt, but story wise the logical sense of time should be long before preparation, else it'd be weird being occupied by trivial things(10 and 20 hours estimated realtime gameplay respectively for both expansions) while there are more pressing matters ahead.
Imo the game itself doesn't have logical sense of time for the sake of gameplay which is understandable, I prefer it that way than having timed quest. NPC will wait as long as it takes for Geralt.
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The devs are right. It's a huge undertaking that would take months to make, i mean imagine: you would have to change the entire world to fit two endings, one where nilfgaard wins and the other one where redania wins. Not to mention edit everything that any character says to keep it in theme with the new world state. So X contract has a piece of dialogue that says "ooh, this war is going to be won by redania i'm sure of it" they would have to find it and re-voice it. I would much rather they spend their time creating new content for us to see, than spending time on a curiosity "which is what this is".
That being said, i still think the ending needs a re-work. It does not provide enough closure for the ending of the story from geralt, which we have been following for years. It just shows you succinct information on what's happened next and leaves you hanging. I mean i expected you meet your friends, and have a long dialogue sequence with every important character like Yen, Triss, Dandelion, Zoltan, Lambert, Eskel you know the main main characters. Something like the dragon age origins or inquisition epilogue, where you talk to everyone to see what happens and what they'll do next.
Also, would it really be so hard to modify kaer morhen to have yen, or create a new world map area called Triss's home in kovir, where you can only fast travel to your house where you can meet triss, have a romance option to kiss or have sex with her, and then be gone to finish the other stuff?
We don't have enough resources or knowledge to assess what they're capable of, all we have is speculation which could never come close to the actual amount of time, money, and whatnot to make any changes happen. What we see as difficult might be piece of cake for them, and vice versa.
Some of the world states might happen even before the epilogue, right after the quest Reason of State, Radovid, Dijkstra, Roche/Ves could have died and they're important characters that will of course change a lot of things that might reflect immediately onto the world.
Regarding voices, can just use the generic voices, the ones that do not talk about the states of the world, but insulting Geralt or saying their own jokes, or just fart, there are quite a lot of variations, repetitive yes, but it serves its purpose.
New area can be made similar to Vizima, it only has one signpost, it has the feel of being somewhere but not part of the other regions. For house in Kovir or even Nilfgaard, no need to make a huge map for Kovir, can just make it like the way they did with Vizima.