You know, it would be really cool to see how Nintendo would develop an MMO set in the Mushroom Kingdom, and other places in the Mario world. They have shown that they not only make E-rated games with SSB and Twilight Princess. The gameplay could echo WoW, but it could play like some Zelda games such BotW.
Now, for the races, I'd say that Toads, Yoshis, Piantas, Monkeys/Apes and even Humans could be on the 'good' side, while Koopas, Dry Bones, Goombas and Koopalings would be on the 'bad' side. For classes I'm not really sure but maybe they could be like the powerups, or you'd just use diffent ones throughout the game?
Edit: Non-Yoshi players are able to ride Yoshis like how the Druid Travel form is. Also Koopas might be neutral, as there's often Koopas living in villages who have nothing to do with Bowser. Humans could be optionally evil too I guess. Also, maybe Goombas shouldn't be playable? They have no arms lmao
I don't think the game would have much customization, but it'd still make for an interesting MMO. Nintendo could team up with Square Enix if they really wanted too!
Thoughts?