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!
Xephyr or Xnest sandbox break screensharing, global shortkeys.
You've just confirmed obvious. No way to improve security without breaking changes. And you demand mostly nontechnical users to blacklist applications. That's a recipe for disaster.
