Another one person MUD project is Bo Zimmerman's CoffeeMUD. It's been around for some time. See http://www.coffeemud.org/
I always felt that a MUD would be a fantastic design approach for a graphical MMO. Specifically, start by creating a text (MUD) version of your world and the basic interactions, and only when you've created a compelling story and experience with words do you translate that into visuals.
It also would be pretty intriguing to create a MUD that could work in multiple media: text, 2d graphics (think point and click adventure), and perhaps 3d graphics. Theoretically it would be doable. A pretty interesting advantage would be that an MMO with a text option would really open it up from the perspective of accessibility.
There's also some good interviews with him on the various Elixir podcasts. I'm blanking on the one he was recently on, but I suppose listennotes.com(?) might help out.
There's a 1 man developer who wrote not only his own MUD but tools to create and find MUDs to play[0].
It's all written in Elixir and open source[1]. He's given a couple of talks[2] about it and does some living code on Twitch.
I have no affiliation with the author but I did watch some of his talks just for fun.
[0]: https://grapevine.haus [1]: https://exventure.org/ [2]: https://youtu.be/Ef_fG5pd5TY?t=36