My apologies, but this is a bit more generic of a suggestion than specific items.
I definitely have gone to the index many times and have come up empty handed when looking up various terms I would think should be obvious in a core rulebook index.
I think the index in general needs to be quite comprehensive and error on the side of too many entries than too few. That said, shouldn't virtually every noun and verb that's associated with specifically with core playing in PF2 be in the index? And, yes, I mean that literally, but please note I'm trying to emphasize those words that would be directly associated with PF2.
Again, I apologize for being critical without giving specifics, but I ran into index issues more often than I'd like with the book. It seems that the default was to opt for fewer entries than more.