So I have a pretty unique idea : What if there are alternate planes of existence that are created when a new intellectual property is created and becomes popular, seems simple  enough, but  here is where it becomes crazy mind-numbing. it's timeline is like a tree, there are one or more main timelines ( Which I shall call trunks due to it's similarities to a tree.) the Trunk(s) are the canon in a story. large "branches" are what if something different happens during a major plot point/ plot line (E.G what if Voldemort won the battle in the end, what if the empire took over the galaxy etc.) and smaller  branches are smaller events respectively (will I get vanilla, Chocolate, or strawberry ice cream at the store.) But, not only that there is a collection of non-canon realities called "The crossroads" were fictional characters that would not meet due to being in a different fictional universes could meet. the crossroads are also capable of creating realities by itself , as it is it's own collection of "trees" or timelines. No two are really Quite the same no mater how identical they may be at first glance, as there has to be at least one tiny difference in a time line. We humans can not enter, but however we can look inside with three tools: an appliance, an operating system, and the internet. Crazy that something we use every day could hold a multiverse.(if your roleplay involves a portal opened from earth to the crossroads, you could use it if you like.)we humans don't know this but we can manipulate it indirectly. we write new alternate branches(fanfiction), we can cause several (flame)wars, even the skilled can code new things into existence. but the same can be said about deities, so the layers just stack up right now.  
please add in the replies if you have a idea to improve or fix anything.and Just to explain this, it feels as if I wrote a short story (and in a way I did.) one heck of a metaphysical  Intro I'd say. I hope you have fun and go nuts with it.