I personally don’t have any experience of using anything but a keyboard and mouse to control WOW. But I have worked with people that found using a standard mouse uncomfortable (mostly due to arthritis) and they had no issues with either trackballs or vertical mice. A quick search turned up a mini wireless trackball that looks like this:
I imagine this would help if you suffer wrist pain but won’t do much if you suffer finger and thumb pain.
Addons like ConsolePort can help you use a controller if you find that more confortable. Shadowlands is bringing official support for the Xbox Adaptive Controller which will make it easier to use alternative control methods, such as foot pedals. You may find the article linked below interesting (although he does still use a mouse and keyboard).
Edit:
Just stumbled across this:
I don’t know if it would be worth the effort to set up, but a Microsoft Kinect is probably pretty cheap to buy these days. And you could play WOW by waving your hands at the screen like a wizard.