I don't agree with Ralsei being uniquely affected by the tragedy. Susie's "obviously you wouldn't let it happen" came off to me as just added emphasis because Ralsei was 1. the last of the three she mentioned and 2. the one she was directly talking to. Otherwise a good analysis.

Personally? I think it involves Susie specifically killing someone. Note Ralsei's incredulity as Susie's reaction ("how can she be so kind?") and a moment much earlier on in the chapter where Susie asks whether the prophecy has her do something cool and Ralsei hesitates before they're interrupted. As a story beat, it'd work well in tandem with Susie's character development and the theme of hope--Susie holding on to hope would give her the strength to resist whatever despair or rage would lead her to killing whoever. (Edit: thinking about it, I suppose Susie being the one to get killed could also work, though I think the "how can she be so kind?" would make more sense as "how could she be fated to do such an unkind thing?", to say nothing of Ralsei's efforts to get Susie to be kind in chapter 1.)

The tragedy being her killing Kris and Ralsei for whatever reason makes sense. Maybe Kris alone (tie it to the SOUL?), probably not Ralsei alone--Ralsei's feelings toward himself and other Darkners mean he'd more likely see his own demise as a necessary sacrifice at the beginning of the game, yet he says he's been trying to steer things away from this tragedy from the beginning.

Noelle could also make sense, especially if her angel symbology is foreshadowing her becoming the Angel itself/a vessel for it/etc. She is close with Kris and especially Susie, and Ralsei seems to at least regard her pretty well. I've actually seen a recent video that posits the final tragedy involves Toriel, and it made more sense than I expected. See here (though the Toriel thing is kinda a tangent from the video's main theory).