Pie menus are awesome. Specially when each item can be executed with gestures. I use blender and it has an awesome implementation of Pie menus. And also highly customizable. The idea is instead memorizing a bunch of keyboard shortcuts, you can just create pie menus with 8 items and call it with a hotkey you assign to it. So instead of one hotkey invoking one command, you can get 7 more with gesture based pie menus. With gestures, it becomes muscle memory pretty easily. This also minimizes the use of the UI, meaning you don't have to move your stylus to the corners of the screen and just stay inside the canvas. Here's a screenshot of a blender pie menu for example: http://imgur.com/mlPJREM Reproducible: Always
I think it would be good to think about features with how they will help you work. A pie menu might be the best solution for it, but it also might not. It sounds like you want a customizable pop-up where you can put various actions into it. For gestures right now we only have pinch and zoom, and two finger panning. We would first have to create the ability to add other gestures, which I am not sure how difficult that would be.
Indeed, Kwin, especially wayland version, should convert all contextual menus into pie-types and on "neutral", desktop space it should replace application launcher by pie-menu that's being filled from .desktop-files & appdata which would also allow adding custom actions. As for gestures, it would be nice to have generic gesture-recognition framework that would work with hand-on-touchpad, stylus, mouse-movement-with-pressed-modifier-button and gamepad's-joystick gestures and could bind them all just as another hotkey. *** This bug has been marked as a duplicate of bug 123284 ***