I've been following the thread and I've been hesitant to chime in, just because everything I have to say is pretty much negative about it and I'm not one to try to discourage someone's ideas or creative process, but there is a lot I have to say about it so why not... it's a discussion forum, right?
Any game I've played that was a large hodgepodge of different themes, systems, source material, etc., has been a rather unenjoyable, cluttered, buggy, and unbalanced mess that tried too hard to focus on too many unrelated, competing systems, and rather than any single thing in the game being properly fleshed out, the focus being spread so thin made a collection of half-assed systems that didn't work very well individually as a result, nor did it work together well as a whole, either.
Even within the Final Fantasy theme/genre, of which I've been a part of since '99, I've seen dozens of games within that come and go, and for a time there was a bit of a push or inclination toward many of these games to try to include pretty much everything... having areas from every entry, having classes, materia, magicite, and so on all side by side. My previous description of the cluttered mess also perfectly describes the games I'm describing now, and that's even within a single theme. The problem is compounded when you're trying to mix a medieval fantasy wizard, with a super saiyan, with mega man, with a vulcan with a dragonrider, with a vampire, with a terminator, with a smurf, with a moogle, with a jedi, with a demi-god, etc.
Even if you figure out balance and how to translate all of those different concepts into a system that doesn't just grossly misrepresent what you are trying to play, then there's the issue of the work involved.
The MUD idea being described would be easily 10 times the work of a traditional MUD, and it would take at least that for any of the systems to be accurately represented. You need a magic system with a mana pool for any magic users, you need a blood pool system for vampires, you need to translate a Mega Man like character's health and energy representation in a manner different and scaled differently to the wizard along with making sure that the E tanks used to regen his health are separate from how other characters do. Then, you need to figure out a system of balance with regards to a force user's ability, but it needs to be a separate system from magic otherwise all you've done is cheapen either the magic system or the force system by just restringing a few things. And then you need a system for power level for the Saiyan's, with the ability to train to increase power level, and again, this is a separate system from Mega Man's energy levels, wizard's mana pool, vampire's blood pool, etc.
Who would be motivated to put this amount of work into so many completely different systems? Even if motivated, who would have the time? You'd practically need an entire team of coders (which come with its own set of issues and difficulties), but most MUDs can't even seem to hold on to more than a single coder and plenty have issues even holding on to 1.
And well, if an insane amount of code work isn't put into each system individually, then you have a watered down mess where nothing is represented like it should, and I can promise you a would-be Jedi or Saiyan rolls into this game and finds the system they want to play cheap and under-representative of that class or race or whatever (especially compared to games that specialize in the theme and where they could much easily play a better coded version of that), they're going to either immediately leave, or raise hell and whine about how on X or Y MUD the system works like this and how come on this MUD they don't have A, B, or C.
While the general idea of a mulit-verse MUD where literally anything is possible sounds neat on the surface, once you start boiling down the reality it simply falls apart.
With hobbyist game design, you pretty much have two choices... you can narrow your focus and do a handful of things very well, or you can spread yourself too thin and do a whole lot of things (at best) mediocre. If I want to play a Jedi, a Saiyan, a Vulcan, an X-Men, a Wizard, etc., why would I want to play a half-assed, mediocre version of it? Is being able to have my Jedi fight a Saiyan in a cluttered, messy game with likely equally messy and cluttered combat really worth playing such a watered down version?
This idea would be much better suited toward a "talker" type of "game", whether it be chat room roleplay or a situation where all game mechanics are simulated through roleplay or emotes.