So... I've been reading through all the NPC dialogue (thank you
@Shademp for extracting all of that!) and... a good chunk of the "added" stuff I have in
this post actually is in the game. It's just... normal NPC dialogue that can be easily skipped over in a playthough. If your impression of CC is only from watching cutscenes, you'll miss all of it. Same if the playthough is trying to rush through the game to get it finished as soon as possible.
As for what is in the NPC dialouge... so much lore is there from the OG. Shinra is... as leaky as a sieve when it comes to a lot of "classified" info. I don't know what the Turks are doing, but they really should look into what SOLDIERs are actually talking about on break. Especially if it's the scientists in the SOLDIER Department (not the Science Department). Maybe it has to do with how all those scientists were deliberately transferred (or are thankful they got headhunted) out of the Science Department because they really don't like Hojo. You've got stuff like the rumored existence of the Black and White Materia (and what they do!), how what the Science Department thought was an Ancient actually wasn't, that SOLDIERs are injected with mako, that Hojo is the one to do the operation that makes people into SOLDIER. Humm.. what else... that Gast, Hollander, and Hojo were the ones heading up the Ancient studies program a while back, the Ancients knows how to talk to the Planet and control it's power, the Science Department was researching making SOLDIERs of their own and Gillian was working on that project. And... how Banora was funded entirely by Shinra. They're also definetly aware that Lazard was a sane boss compared to the likes of... basicly everyone else in power at Shinra.
Never mind that you've got scientists who are really angry with the Science Departmetn blabbing about Cetra Lore (that sounds... a lot like parts of the hologram in Remake) and what Hojo did to Ifalna (who is known to be the last surviving Cetra!) and how that justified them releasing a bunch of experiments all over Midgar in the hope they'd kill everyone of. And they're saying this in the Exhibit Hall that is filled with visitors to the Shinra Tower. No, I am not making that up.
You've also got scientists in the actual Science Department who think everyone in SOLDIER are the Science Department's puppets and are just meatheads (which... definetly isn't the case given the SOLDIER Company Grapevine).
Also, there's just... tons of dialog on the SOLDIER floor that shows Zack stepping into Angeal's shoes in the SOLDIER department and being the guy everyone goes to for reassurance no matter what their rank is. CC definetly was setting him up to be the heart of SOLDIER if Nibelehim hadn't happened.
Really, I don't know how Sephrioth never connected the dots about some of this stuff. Because he grew up in Shinra! To say he comes off as deliberately not wanting to think about this stuff is... an understatement. There are mentions of him being in the SOLDIER Records Room digging up the history of the science department... but given that people already know some of it (more like... a lot of the parts that actually matter), I don't know what he was expecting to find that he wouldn't already have gotten the chance to hear.
So yeah, something Crisis Core actually got right was... lots of worthwile NPC conversations when it comes to world-building. Which is often one of the first things anyone speed running the game skips...