We will have to go back to the source material I guess. Im fairly certain it's a sexual aura/magnetism and it made Triss 'uncomfortable'. The problem with doing a direct sequel to the witcher 3 though is the fact that you have to take into consideration all the different ending possibilities and create a different narrative for each one if you want to stay true to the lore you've created with your choices. That's where things start to get far to complex. For example is Ciri alive or dead, empress or witcher? Is Dijkstra ruling the North or Radovid? Who is ruling Skellige? Lambert/ Metz alive or dead? ....etc, The possible solutions if we use Eskel as a supposed example of a possible protagonist is. The game needs to be set in other lands far away from the ones already used like Kovir and Poviss, Zerrakania and Ofier, that way you don't really have to address those issues deeply. They simply put in a world building cut scene like the one with Voorhis and establish if Ciri is alive or dead and is Geralt with Triss or Yen for example. That way they can make cameo appearances like Sile and Letho. Or have an Older Eskel play the protagonist in which you create an entirely new political narrative. Otherwise a prequel with a young Vesimir would be far easier.
I think they could easily accommodate the three political endings. Just stay away from politics. Besides Geralt, all of the other Witchers avoid politics whenever possible. Just change the soldiers to Redanian if Djikstra or Radovid win, and Nilfgaardian if Emyhr wins.
Or, alternatively, make them Nilfgaardian, no matter what. Sapkowski and C.D.P.R. have given the impression on multiple occasions that Nilfgaard is this unstoppable force that will eventually take over the world. And, since the throne belongs to Ciri, and her child is destined to conquer the entire world, it makes sense. Personally, I think this would be pretty cool.
Yes...it would render our choices moot, but Andrzej Sapkowski has already done that for us, when he shared that prophecy, in one of the books.
Is this actually possible ? As far as I know, even if you tell him to stay, he still leaves.
"staying in Kaer Morhen" doesn't mean "living there forever". He would still go out on the Path, but come back during the Winter. If you don't convince him to, he'll say that he's never coming back, period, due to the sadness that the old fortress brings him.