Eli,
I have not created a perfect package, I have just shared something I personally use, and that works for my purposes (to separate files I want to backup from those I do not want; and configuration files which I share among machines from data files which are generated in the machine, like recentf or Emacs session files). I emphasize the idea that Emacs must adhere to the XDG protocol, as almost all other applications are. Emacs does so much more now than twenty-five years ago, when I started using it, and the configuration files have also grown concomitantly.
About the adoption, I think it is a non-issue. At the very least, user-emacs-directory should be changed to ~/.local/share/emacs, if it exists, and keep on being ~/.emacs.d if not. As far as I know, no package has "~/.emacs.d" hardwritten, so the transition should be painless. In the same spirit, user-emacs-init should look first for (locate-user-config-file "init.el") then ~/.emacs.d/init.el and then the usual choices, as detailed in the Emacs manual.
(At this moment, ~/.emacs takes precedence over ~/.emacs.d/init.el, which is annoying. Debian and Fedora tend to install a .emacs of their own, and a pretty useless one. I have been fooled once, surprised my configuration in ~/.emacs.d/init.el was not called.)