@HelloProject
I'm going to continue my thoughts from the HeadWiz thread here, because they are more pertinent to the general building of this MUSH than broad HeadWizzing behavior. The last post I made spoke of power levels, advancement, and disparities between source materials. Around here, most people are pretty comfortable with wild power level disparities, like I said. But -- and I've discussed this with @Benediction at length -- even though I personally enjoy being the Big Guy who can whip on groups of lesser people, it is simply the truth that it's not fun to be functionally unable to contribute to a scene.
So I'd like you to consider a design decision made by Super Robot Wars MUSH: Their mantra when constructing their theme was / is Total Theme Control. What this meant is that while they were attempting to construct a game in which many of the Super Robot Wars notes would be hit, they weren't married to the canonical lore and weren't afraid to rip it out and replace it, or otherwise change it up. The Spiral Empire wasn't just The Bad Guys, even the awful space monsters that have showed up have reason and decent rationale for the things that they're doing instead of being a generic boogeyman to be punched.
Gaijin Chaos Z isn't the story of Son Goku or of the Saiyans. It isn't the story of Naruto Uzumaki, and it isn't the story of Yusuke Urameshi or Usagi Tsukino. Those characters are all a part of the story and they're hooked into existing backstory elements but they're not and shouldn't be the end-all-be-all because it's a MUSH and not their manga.
So how do you deal with Saiyans being Objectively Overpowered? Same way SRW makes Spiral Knights not objectively overpowered: You assert Total Theme Control and just take the overpowered bits away, in this case zenkai. Why? Because it's fine for a Shounen Protagonist to have that in their manga, but the MUSH revolves around more than the Saiyans. So then, what makes being a Saiyan special? Well you know, turning into a Great Ape is actually pretty god damned cool if you ask me. Just don't have it get taken out by tail-cutting and bam, you've got a race with a Cool Power Up that scales but doesn't double their power whenever they get beat up.
Sailor Moon presents a similar problem. At the end of the day, only Sailor Moon and her omnipotence crystal really matters. But it's fine -- even preferable, I think -- to change things like that!
One thing I think you actually need to seriously knuckle down and think about is transformations in general, because this is one of the worst pieces of escalation you're going to have to decide on the appropriateness of. Pretty much all of the shows you're using include an endless escalation of new, ridiculous transformations with new, ridiculous attributes but these transformations are almost inevitably only attributed to main characters.
Do you rip the transformations out? What about characters who have no natural transformation (Like Krillin, or most characters who aren't Jinchuuriki in Naruto), what do you do for them? Do you downplay the transformation aspect and simply let them ascend to a higher tier naturally?
On a less individual, power-based level, how do these various settings tie into one another? How does the Dragon Ball afterlife interact with Yu Yu Hakusho's metaphysics?
In summary, you've got a lot of decisions to make. One of the best tools you could have for answering them is to assert Total Theme Control like SRW did and unapologetically tell people that there will be changes for any number of considerations, and things might not look exactly like they expect.