Alright, I am one hell of a wet towel, and probably not the type of person this thread is suppose to attract. But let me just explain why this idea sounds like so much fun but tends to turn into cancer really quickly. 

I think we all know why this idea sounds great already. Because due to the magic of the internet, it is the one place where you could be literally anything and interact with anyone. Like you mentioned already, you could be a super sayan and surround yourself with waifus, or something too strange for most other RPs. In an idealistic world, everyone would get to do what they wanted in the same RP. And that's magical. But when people try to execute ideas like this, it quickly becomes a mess.

The first problem you have is the power scale. You have a lot of different worlds with different levels of power. The entire cast of dead or alive couldn't hope to stand a chance against a sayan, let alone a super sayan. If said super sayan happens to also be a dick, he could easily blow up earth after taking his harem safely to another planet. Sewer surfing ant can't do jack all about it. Even if you put a "no douche bags aloud." rule in effect, you're still going to need to think of challenges that are worthy of a super sayan, but still able to be completed by a sewer surfing ant, slime girl maido, or whoever else joins this RP. What happens sadly is that your RP groups develop into cliques, and they never really interact with each other if they can help it. But you'll have a lot of in fighting before that, and some people will just outright leave the RP. It can also be hard to decide which rules from which world to enforce. 

Closest thing I got to playing a youtube poop RP was a crazy feudal japan era RP. I had a fire sword that when swung into an icy tundra, it turned it into a beach, complete with sunbathing supermodels, surf shops, etc. What I've found is that in a place like that where there is so little reason, it can be difficult to tell a story that players can take seriously. Also, it at times turned into a war to see who could be more silly and "funny" and it just sort of evolved into some nonsensical nightmare.

I think trying to do something so open ended is a very bad idea. Something like this might work if you can get everyone on the same page. Limit the randoms, find some reason for super sayans to not be able to wipe out planets if you plan on allowing them, and find a reasonable level for the silliness where things can be funny, but not so absurd that they break the world.