Well, I would suggest using Ash instead of Tree. Lords of the Ash would be pretty good. Roots of the Ash might be good if you want to go for the world tree feel.
I think if you can get something in about hanging from the tree, that would be good. Although Odin was the only one I know of who hung himself, it is supposed to be the Gallows Tree after all. Fruits of the Ash might be interesting, but honestly, I don't think enough doomy-gloomy

You might do something with Ratatoskr, as he runs up and down the length, and he would probably see all those races along the way. That and you have the maidens already.