Using Toussaint is also not just randomly picked and the reasons for it are most likely the same reasons why we won't see references to the main game.
It is a vassal state by Nilfgaard, that's true, but it's one of the few, which is more or less independet with an own rich culture and own problems aside from the world problems. You can't even really leave that place, if it is winter, that's how locked-in it is. They have to pay taxes and provide ressources to Nilfgaard, but as far I know that's it. Might be wrong about that, but I believe they don't even support Nilfgaard military (troops).
I wonder if those people even care, who would win the war, because nothing really would change for them.
They also have this "romantic" knight thing, people always see in medieval settings (or the guy who challenges Geralt several times in the game). A knight is a gentleman and has to rescue the princess, stuff like that.
A little bit like their own world and maybe that's exactly why they chose it, to have more freedom again and not being limited by something.
In HoS they used the Ofieri, a culture and nation we barely know
anything at all, so they had the freedom to create for example runes to improve the gameplay.
Even their language is (most likely) just made up by CDPR.
So choosing Toussaint as the next (and so far last) location is already excluding any connection the main game and therefore their characters and (side)stories from the start.
The only thing I could imagine how it could still work, would be, if they make it some years into the future, after everything settled down already, but even then they have to make some things even and references like who won the war have to be so vague that they are going to be pointless anyway.
If they want to consider all decisions and results, it would be a pain in the ass to connect everything to Toussaint, not because of the amount of decisions, but because they vary quite a lot.
Triss would be the easiest, she is in Kovir in any case. Just say she has to deal with some diplomatic issues in Toussaint (though why should she deal with Toussaint and not Nilfgaard directly?). If Geralt wasn't there already, he would just accompany her.
Yennefer, well, if she is with Geralt, not much to explain then, she just went with Geralt to Toussaint on their journey, but if she is alone, what reason would she have to be in Toussaint? She has no connection to it except for Fringilla and well, let's say that's not exactly the friendliest one.
Ciri. Uff. If she is the emperess, it's easier, she wanted to see one of her vassal states. A witcheres? She was just nearby and heard Geralt is around. She is dead? Well, shit
From the other friends only Dandelion would be there, as he is the only with a real history with the county, I don't want to spoil that for those, who haven't read the books yet (what you are waiting for?

), but you are in for a huge surprise.
The other witchers, well, maybe they are there? Maybe not. The continent is huge, really huge and meeting several old friends at the same place in such a huge world is really unlikely.
Other sorcerers? Yeah, but only one makes sense (and because she is alive...):
Fringilla Vigo. If she isn't in that expansion, they should have killed her along with Sheala de Tancarville.
The war? Well, they could just "skip" it, as I said, not much would change for them no matter who won, but it would be quite weird, if nobody is even mentioning its outcome. Depending on what the expansion will be about, if you are talking with characters, who are involved in politics, it would be quite strange, if they don't mention it at all.
The other decisions don't have any influence on Toussaint (Skellige, fate of the mages in the northern realms, Keira and so on), I guess.
I am not saying it isn't doable, but considering that they didn't choose it on a whim, I think it's a pretty safe bet to say that Geralt is the star of the adventure and except for Fringilla I don't really expect anyone else. It would also give Fringilla the attention, she deserves. Probably the most underdeveloped book character so far and had pretty much no purpose in the game.