On another tangent, I wish I had known Common Lisp back when I cared about MUD implementation enough to actually pour a few hundred hours into it. Of course you'd face the problem of the extension language, at least in the naïve implementation, being entirely too powerful. Still, it could potentially make it easy to script truly interesting custom object behaviors.
If I were to make a pass at it now, I'd try to structure the game system in such a way that it could power a roguelike or a MUD, since roguelikes have considerably more legs these days.
Not to mention that a graphical MMORPG is one of the most expensive online games you can develop. Whereas a MUD (a text-based MMORPG if you will) can be created by a lone dev or small hobby team (not to say it's simple, no multiplayer game is, but it's at least achievable).
I think it makes a lot of sense that the remaining MUD players are those that have the most complete affinity for the system. While MUDs draw a technical distinction between making and playing, I think psychologically the lines are very blurry, because part of playing is making.
One reason Minecraft really took off is that embedded in the choice to "live" in an expansive virtual world is a desire to affect that world. It mirrors the malleability of our world but on a larger scale. Climbing up an RPG power curve is only a pseudo-satisfier of that desire; you're not actually changing the world, just changing your position in it. Actually being able to modify the world you occupy is the base desire. In that respect, Minecraft is really the spiritual successor to MUDs.
A really curious thing about muds is more people seem to want to make new ones than play them. There is probably about one unique mud per active mud player these days. Most have no one playing them though.
And the established muds have a hard time finding people to work on them, since they are gnarly legacy codebases based on deeply out of favor programming paradigms. Much more fun to roll your own platform, even if it never gets played.
I wish people would just pick one of the dozen or so active muds and play it. Mud client programming is a great outlet for recreational coding imo. The line between my character and the code I've written to manage it is very fuzzy in a good way.
Plus everyone is always talking about how great lua is. Mud scripting is mostly lua, so it's a great way to find out that lua is just as bad as any other language.