A set is a collection of things, each of those things could be anything. Now, what if those things are other theories? That would mean that we can represent all other theories within set theory, and thus set theory is the most powerful a theory can be. There can also be theories similar to set theory which can have all other theories represented inside of it... what if we call those "limit theories". All of them would have to have equal power since the others could be represented within them.
What do you think some of the other "limit theories" could act like?