(I have a fix for the "multiple files" problem, which I'll be sending to xdg-utils upstream when I can.)
Hence it would seem sensible to have a new default for dired-guess-shell-alist-default, which matches any file and returns the appropriate command for the OS, probably doing some testing to see which commands are available on a GNU system ("xdg-open", "gvfs-open", "see"…).
The user then only needs to understand and configure one command-launching system, and the coverage of different filetypes is much more comprehensive in most such systems than in dired-x.el.
An advantage to Emacs is that this system could then be used elsewhere. For example, org-mode already has a similar, separate system, org-file-apps, which might benefit from building on dired-x, and only retaining its own configuration where it's desirable to open a file "internally" to org-mode.