The only convincing multiverse concept I think will really succeed is one that is a single roleplay with multiple worlds built into it already. Players can be whoever and have whatever powers, but they all have to fall under one single universal set of guidelines. Magic, guns, swords, doesn't matter, but there would need to be some sort of basic leveling system for actual combat. It would need one main overarching plot, with separate plots and events on top of it (that complement it) for people who want to do their own thing, as long as it was moderated by a GM and Co-GMs. 

Sounds good, right? Well, it already exists. Holy Soldier is already running a video game version of this [i]exact[/i] concept, down to the rules, leveling, and multiple storylines...[url=https://www.roleplayerguild.com/topics/156841-project-crossover-video-game-crossover-rp/ic]in Casual[/url]. For over a year. And it's active. And he's doing a great job.

So, why make a site-wide multiverse section when, if you put the work in, you can do it yourself in Casual (or Advanced)?