I think an MMORPG like WoW or LOTRO can be playable with one hand, either with a controller (if it can be set up to work) or a special gamer mouse with lots of extra buttons.
Creating a class that's playable with just one hand will be heavily abused by people who don't have disabilities and it will break the game.
What I see as an only possibility is for the developers to adapt the game to be compatible with devices for gamers with motor disabilities or whatever. It has to be an investment on both sides where the developers increase the compatibility with devices and users purchasing the devices in question.
But just creating a class or adding a simplified control scheme looks unfeasible to me. I currently play LOTRO and WoW and I'm pretty sure I can play both games with just one hand, whether it's the keyboard hand (with minimal input for mouse where I would need to talk to NPCs for quests and other UI navigation) and with just a mouse, holding down LMB and RMB or Mouse3 makes your character walk, RMB starts auto combat and from then I will just have to click each ability on my ability bar. That is if I'm using a standard 3-button mouse, which is what I have, but if I were to have a gamer mouse with lots of extra buttons, something like this:
https://www.amazon.com/Logitech-Gaming-Backlit-Programmable-Buttons/dp/B0086UK7IQ/ref=sr_1_3?keywords=Gaming+Mouse+Buttons&sr=8-3
I could easily imagine myself playing without ever touching the keyboard. So more or less it's a personal investment. I've seen videos of a lot of disabled gamers that have found ways to work around their disability and still be able to play the games they want, only because they wanted to play really bad... so they found a way.