http://simmesimme.github.io/gnome-pie.html
And especially his delightful thesis work:
Trace-Menu:
I really love how the little nubs preview the structure of the sub-menus, and how you can roll back to the parent menu because it reserves a slice in the sub-menu to go back, so you don't need to use another mouse button or shift key to browse the menus.
Coral-Menu:
That looks like a nice visual representation with a way to easily browse all around the tree, into and out of the submenus without clicking! I can't tell from the video if it's based on a click or a timeout. But it looks like it supports browsing and reselection and correcting errors pretty well! (That would be something interesting to measure!)
There's another useful law related to Fitts's law that applies to situations like this, called Steering Law:
https://en.wikipedia.org/wiki/Steering_law
The steering law in human–computer interaction and ergonomics is a predictive model of human movement that describes the time required to navigate, or steer, through a 2-dimensional tunnel. The tunnel can be thought of as a path or trajectory on a plane that has an associated thickness or width, where the width can vary along the tunnel. The goal of a steering task is to navigate from one end of the tunnel to the other as quickly as possible, without touching the boundaries of the tunnel. A real-world example that approximates this task is driving a car down a road that may have twists and turns, where the car must navigate the road as quickly as possible without touching the sides of the road. The steering law predicts both the instantaneous speed at which we may navigate the tunnel, and the total time required to navigate the entire tunnel.
The steering law has been independently discovered and studied three times (Rashevsky, 1959; Drury, 1971; Accot and Zhai, 1997). Its most recent discovery has been within the human–computer interaction community, which has resulted in the most general mathematical formulation of the law.
Also here's some interesting stuff about incompatibility with Wayland, and rewriting Gnome-Pie as an extension to the Gnome shell: