The only characters off the top of my head (not counting the Strawhats) who could possibly replicate the emotional impact of Ace’s death on Luffy and the readers are: Sabo (not going to happen imo), Vivi, Garp, Shanks, Coby, Boa, Law, Carrot, Yamato, Tama, Momosuki, and maybe (and these are big maybes) Rayleigh, Marco, or Hiyori. I am leaning toward someone Luffy loves getting captured rather than killed. It feels a bit cheap for Oda to pull an Ace death 2.0.
As for other deaths:
I think there is a high chance some of the scabbards will die.
I also think Kaido will die after doing something heroic.
Orochi might get whacked after he gets defeated. However, it may not be in a spectacular manner. For example, Cipher Pol getting pissed and shooting him.