What was the weapon Arya wanted Gendry to make?
Read somewhere that it is a re-creation of a Dornish weapon. Arya is on record of being a fan of the Dornish queen Nymeria, thus why she named her direwolf that. Dornish warriors are known for using short "throwing" spears as well as short sabers. The Dorns were also the first to kill one of Aegon I's dragons after hitting it with a spear. While training in Braavos, Arya was shown practicing with a short spear, but has yet to use one in battle.
Theory is Arya ends up using the weapon to kill either the Night King or his dragon.