What role could the dagger play in the show?
It doesn't look like the primary purpose of the dagger is to revisit the catspaw assassin, because Bran as the Three-Eyed-Raven does not care about the trivial moments in the past with the war against the Night King coming. Littlefinger possessing the dagger openly to Bran makes it seem unlikely that the show will alter the book storyline and make Littlefinger the person who tried to kill Bran -- both because Bran would have seen that and because Littlefinger seems unlikely to harm Catelyn's children. However, the efficiency of the show's storytelling this late in the narrative means the dagger has to be back for some reason.
The dagger's chain of ownership -- from Littlefinger to Bran to Arya Stark -- suggests that the dagger is going to play an important part in the future of the show. Is it once again Littlefinger's scheming is at play or an instrument to test Littlefinger's ability to adapt to chaos? Ever since the dagger and Baelish have been in the same place, the dagger is a symbol that Littlefinger's working against the Starks. In the first season, he used it to imprison Ned. Season 7's scenes have Littlefinger using the gift as a way of failing to manipulate Bran. The dagger pops up again with the three Stark children, prompting Sansa to question Littlefinger's motives. Then Arya uses it to spar with Brienne, a silent power shift between the two Stark girls that Littlefinger wants to use to his advantage. The dagger's presence means Littlefinger is working against the Starks, yes, but what if the answer to Bran's question, about who owned the dagger in the first place, isn't "Littlefinger, but "Jon Snow"?