TL; DR: The first
important death will be Vesemir; or if the story turns toward them, Eskel or maybe Lambert. However, this wants explanation.
Tragedy is a very special art form, and only rarely is it well done. I believe CDPR's writers understand tragedy and will inform their decisions accordingly. The best description of modern tragedy is by Arthur Miller, in his essay "Tragedy and the Common Man" (1949),
"I think the tragic feeling is evoked in us when we are in the presence of a character who is ready to lay down his life, if need be, to secure one thing-his sense of personal dignity. From Orestes to Hamlet, Medea to Macbeth, the underlying struggle is that of the individual attempting to gain his 'rightful' position in his society.
Sometimes he is one who has been displaced from it, sometimes one who seeks to attain it for the first time, but the fateful wound from which the inevitable events spiral is the wound of indignity and its dominant force is indignation. Tragedy, then, is the consequence of a man's total compulsion to evaluate himself justly.
In the sense of having been initiated by the hero himself, the tale always reveals what has been called his 'tragic flaw,' a failing that is not peculiar to grand or elevated characters. Nor is it necessarily a weakness. The flaw, or crack in the characters, is really nothing-and need be nothing, but his inherent unwillingness to remain passive in the face of what he conceives to be a challenge to his dignity, his image of his rightful status"
I emphasize, "the fateful wound is indignity and its dominant force is indignation... a man's total compulsion to evaluate himself justly.... The flaw is his inherent unwillingness to remain passive..."
Geralt, Ciri, and quite possibly both of them are the hero(es) of this tragedy in the making. It is the injustices and indignities that pile upon them in their search for Yennefer and each other that will drive them to tragic action. This suggests an important, unjust death of a character dear to Geralt early on, though I may be wrong. Geralt's witcher companions, unfortunately, are clear targets for such a death.
Dandelion lives to tell the tale; Zoltan is too indestructibly earthbound. Iorveth and Roche are no longer important: their actions or deaths would be in the background. I do not think Yennefer or Triss would die early. And few other characters would matter.
As to how it ends, it would not surprise me to find one or more endings with either Ciri or Geralt dying, and the other going all "whirlwind of rage and steel" to set her or his death right.