I'm about 10 hours deep into this one and still really enjoying it. I'm sitting at 98% with a few more ideas. I'm in the middle of a final pass through all the areas, but I'm also curious how the NPC's might affect completion percentage. Will the NPC's drop unique items like in Dark Souls? Will the NPC in Bleary Backwoods who mentions judgment seek retribution if I kill one of the other two NPC's? I'm not really looking to be given direct answers, but I am curious how many others have thought about these things or simply attempted to reach 100%. Great game! Looking forward to Brume next.

Update: I've since completed my scan of all the areas, but wasn't able to spot any other secret items. I killed Carlett and the dual-knives NPC, both of which dropped unique weapons, but I'm still at 98%. Also, I have six astral feathers and Estar tells me that if I bring him seven he'll upgrade my weapon. This suggests that I'm missing an astral feather at the very least.