Pretty much all of it was stuff I wanted to do from the start. You can check the Hinterlands Postmortem, in the Cut Content section, and you'll see that three of the five new events we added were explicitly mentioned there. I already had a very solid idea of what should happen in these scenes and how it connected to the larger narrative. I felt very strongly that without them the story of the Hinterlands wasn't as much of a "fair play" detective story as I wanted it to be.
The other two (The Cemetery and The Parade) were already on my mind but not as concretely. Still you can see some of the ideas were already there. The idea that stores close down on the last day (September 7th, the Independence Day holiday) was tied with the idea of letting P and Storm watch the parade going through town, which would give a nice capstone to the bond the two created with each other.
As for the Cemetery... Well, I actually managed to put in a hint about what my idea was for it in T̸̢̳̦̲͕̳̳͛̈́̂̈͒̀̈́͗͆̀̀͝H̸͇̘̭̟͛͂̋̕͝͠Ȩ̸̗̞̗̲͉̤̝̝̒̏̋̌̈́͛͒̾͝ ̴̧̨̛̟̲͓͕̗͈͕̀̍̾̍̓͝ͅÁ̷̟̪͔͖̆́̍͌́̚͘̚͘͝͝N̴̢̡̢̰̍͛͐̄̀̂̀̆͐̇̂͆̀͊̂T̷̛̜͋̓̀̇̔͑̊̉̌̕H̸̪̥͕̯͔̦̖͙̬͇̙̲̫͎̒̾́I̶͇̭͖̥̱͎͇̽̑Ľ̴̞̹̠͂̆̾̽̽̂̓̈́́͠L̸̫̜̙̰̝̫̥̔̋̃̽̀̓ of all places. I already had an idea of what I wanted but, seeing as I didn't have time to put it in, I was a cheeky little shit and slipped it there. Both as a hint and as a reminder for myself about what I thought was a neat idea.
The stuff that really emerged during production were some of the loot drops you can find in the expansion. For a few of them I was already 100% sure what I wanted them to be, but for the most part I developed what they were about quite late in development, sort of as a cherry on top.
Another thing that came up during development was me writing down the history and some of the rites of the Latin tradition. I didn't put that into the game, it would have been very unnecessary and bloated, but that was a late thing that helped with figuring out what I wanted out of the loot drops.