So Sylvanas finally gets her soul back and the first thing she does is shatter it again this time without frostmourne just to add it to a arrow to shoot at the World tree.
Sorry but even if they could create a world tree from such a weak soul, weak as in a soul is nothing versus the power it took to create the tree´s in the first place, why not just do a ritual where the soul is used as power instead of shatter the soul, then add it to a arrow like you would with some random little trinket and then fire the arrow.