Is it intended only for mobile devices or also as an accessibility keyboard?
That is a normal always on top window (movable/resizable) , working everywhere (not just when focused on an input field) + features like different input modes (hover, …), keys like “right click”, ideally customizable layouts, custom keys/macros. Like Onboard, Florence and Windows/MacOS accessibility keyboards.
Related thread: Plasma 6 and Wayland no on-screen keyboard working
An example of MacOS with an advanced setup: blakewatson.com - Writing and coding with the macOS Accessibility Keyboard