I disagree and I don't think it would affect much into main story line. Most of the important choices in witcher 2 was political and in witcher 3 political/millitary stuff is secondary content: "If we want we can impact it". So changes in those areas shouldn't conflict with "game's standing on its own" issue.
Exactly, what I'd want is the decisions I made in TW2 being reflected in:
- The dialogue
- The people I meet (if I saved them/let them live)
- The common folk/people (Witch Hunt or no Witch Hunt?)
- Secondary Missions/Quests
Like for example, I'd like to see if people are now starting to hunt Witches, I'd also like a secondary quest where I might be able to save Saskia (or end her? based on the fact if you picked up the dagger from Iorveth), or meet Iorveth again. Also, is Triss my friend or is she closer to me now? The dialogue should reflect which king is ruling over which land (and/or army) now (even if the country is occupied atm), etc, etc.
I do not want world-changing consequences (aside from the one with the witch hunt/ no witch hunt) but they should be reflected in a way, either mentioned or shown by meeting saved characters or getting an additional secondary quest (the people who haven't saved said character could get a different quest there that might hint at the possible involvement of the character if you had saved him). Stuff like that.
And I think that is not a difficult thing to do.
Good examples are the dialogue with the King in the Prologue of TW2 where Adda is mentioned, Aryan LaValette, the thing with Siegfried in Chapter 3.
A bad example is Triss in the beginning of TW2 (no Shani, no option for "just friends" until much later).