I know that most players don’t have all the old weapons, but assuming there will be some way of collecting old weapons Soon™, this analysis considers which Kingdoms most need new weapons at lower rarities, or alternatively, which Kingdoms will be most difficult to upgrade Kingdom Stars for.
Kingdoms with less than 2 weapons at Epic or lower rarity (i.e. must upgrade at least one Legendary weapon to get to 8 stars):
Forest of Thorns
Kingdoms with no weapons at Ultra-Rare or lower (i.e. must upgrade an Epic to get to 5 stars):
Kingdoms on both lists could be argued to have the greatest need for a new low rarity weapon: Bright Forest, Ghulvania, Khaziel, and Pan’s Vale.
I’m happy to do other cuts of the weapons data if people are interested.