You can use Xephyr or Xnest to sandbox an untrusted or insecure application within its own X11 instance. This gives you the exact same kind of security property that Wayland happens to enforce out of the box for its clients, except that it need not apply to basic desktop components such as the window manager or the desktop panel. You don't even need Xlibre or anything, this stuff has been around for ages. It's not rocket surgery!