Something I appreciated was how much the game seems to remark upon unfinished business, and crossing off the TODO list, and then often seems to further reward it when you go just a little bit beyond. That also continues to feed into the overall themes: The Secret itself being unfinished business to cross off a TODO list. A general sense of leaving the world a little better than you found it, or at least trying to put some things back (almost) into their proper places. (Be courteous to other treasure seekers.) The non-hidden achievements tell me I have a couple I missed to try for. I don't know all that I'm missing in hidden achievements either. I found at least one currently super-rare one according to Steam Achievements percentages. I wonder in general how many you can cross off of Elaine's "post mortem" conversation in general, that seemed enough like a "report card" that I'm wondering if there are incredibly hard versions of the puzzle solutions that "fix" some of the wrongs Elaine mentioned. But maybe Adventure Games have just conditioned me too much to think there are better solutions than the ones I found, and that plays into the themes too if there really is no way to solve some of them with an Adventure Games mindset, that you can't ever actually check the entire TODO list.
I caught at least one other Papapishu (other than the one on Terror Island), I think it was while playing with various bits of fire on LeShip. It does amuse me that they avoided many of the obvious callbacks and jokes, but that one made it in and remained consistent.