^Posting that on my FB page, if you don't mind.^ :-D
I have a couple of theories on the significance of the Catspaw Dagger that's been prominently featured in this episode:
Personally, I think it's gonna be bigger than just "Arya is gonna kill Littlefinger with it" which is a popular theory. I think she will but that's not all. I have two theories on what might happen.
1. The Catspaw Dagger through the heart is the only way to kill the Night King. I think Arya or maybe Jon will have to do this task. Stabbing the Night King through the heart, like how the Children of the Forest created the White Walkers, the same thing will destroy them.
2. If the spoiler leaks are to be believed, one of Dany's dragons will die and resurrected as a wight dragon carrying the Night King. I think it's possible Bran might take this Catspaw Dagger through his heart to become a White Walker (a la Benjen) in order to warg into that wight dragon and save humanity.
I think one or the other will happen.