Nope, not correct)) We want past-epilogue state. And c'mon, this would require changes which will be easily done using RedKit (when it comes out). The only problem in the case of "modding" will be console players being left out
If you want post epilogue then changes are massive including winter time and no war anymore, plus most of the character will not be on the map probably.
Including the sorceresses, Ciri (or does this become a co-op thing if she's a witcher? Empress and dead she left), surely Triss (unless Dijkstra won, but then again she should be already in Kovir). Who's left? Oh Lambert and Eskel left for their own Paths and are not gonna come back to KM as per dialogue.
Dandelion and Zoltan maybe.
I don't get your arguments: it'd still be a frozen world, only a bit later. If as you say, it's doable with redkit and easy than that necessarily excludes all the dozens of different new voice recording it'd entail if done properly. So what would the handful of NPCs add exactly? Say "well?" over and over?
---------- Updated at 12:32 PM ----------
Again I don't want to touch it here...
I said that new dialogue is not necessary.
Adding all the topics which NPCs had throuout the game (except those related to quests and plot) would suffice.
Well that'd be more underwhelming than them disappearing imo.
Your average RPG with no endgame aknowledgement at all.