I don't think there's any reason to fly up to the branch. I'm more interested in what happens if you sail in the direction of the branch and find the place where the branch meets the water. Or is there like a big waterfall that serves as the absolute boundary of the world?

Maybe in a later update, a true endgame can be provided in which you kill the last of the Forsaken, and are granted the right to leave Valheim and go to Valhalla, and the exit can be found by following the Yggdrasil branch.