I decided to take a good look at my wrist and see what range of motion they are capable of.
Please excuse the simplistic drawings, but I think you get the idea. I determined what range of motion I can get from my own hand and decided how to accomplish that for my BJD. The idea of the wrist being a sphere with slots is not a new concept. I did want to make her hands exchangeable, so I am planning to use magnets.
What my drawings doesn't show is how large of a sphere to use on the wrist to accomplish the range of motion while still looking aesthetically pleasing. That I will experiment with in carving wax to see.
Zen & The Art Of Articulating Dolls By Using Balljoints by Twigling
Scroll to the bottom for info on wrists.
Joint Range of Motion
I used my own wrist for my drawing and number for range of motion. Yours may vary.
Has information about degree of joints in relation to position.