<Snipped quote by Techspert>
I read the first post in the OOC, it didn't seem to mention characters needing to fit into the world?
<Snipped quote by DarkwolfX37>
I assumed "Existence" just included everything and that essentially all characters would be allowed. I read the OP and my impression was that it's an RP sandbox for doing whatever you want with whatever characters you wanted.
Existence is every dimension. While most anything is applicable, it'll be difficult to get interaction with other players without an understanding of how your character could possibly end up meeting them. For example, if you choose a regular character in some established universe, there'll be almost no way for them to run into the other characters in MR.
In other words, while the idea of a sandbox isn't strictly incorrect, it's still a sandbox that you're sharing with everyone else =)