You'd also need to reconcile their in-universe rules and make a very clear set of multi-verse laws. Since one world's rules may not mesh well with another's. The bible for Harry Potter no doubt works much differently from the bible of Lord of the Rings as both would the bible for My Little Pony and then as they would be different from Diablo's or World of Warcraft, you'd have to create some system of magic that would bring the three to a common table.

From a sci-fi perspective too, some systems may or would require specific resources from that universe to work. I don't think the crystal necessary for Jedi Light-sabers would be very common outside of the Galaxy Far Far Away since that's only in George Lucas' head and the Spice of Dune is only in Frank Herbert's. So you'd need to build an opt-in from scratch that still flows with the rules to be built by establishing the previous.