Then plan for it, architecturally, so that you can add or enable these features later on, or when you need it. Start with something sensible, appropriate for real-world usage. Wayland was everything but. With X11 the ship has sailed in that regard, but if you start over, don't create something that doesn't even offer these features in the name of security and then let the community come up with 10 different solutions to the same problems in the most uncoordinated way possible.
> Not if you were aware of the current issues with Xrandr and why it is a poor target.
xrandr works exactly fine for setting up your screens. You can pass resolution, refresh rate, position and orientation via command line in a simple and understandable way. It's a great example for a trivial real-world tool that should just exist.
> It is absolutely not trivial, and there are security/usability issues with it. To do this "the right way" would be more or less duplicating the entire KMS API into every implementation. Even XRandr is still missing features compared to that, so it is not an adequate solution either.
This completely confirms second system syndrome. It's only as complicated and non-trivial as you want it to be. I can only imagine what kind of convoluted security nonsense you think of when you say "the right way". Ask the display server for a list of screens and supported resolutions, and then tell it what you want to do with those. Done. X11 does it, Windows does it, macOS does it.