How Do You Know When an Idea Is Big Enough for a Universe?

How do you know when an idea is big enough for a universe? I always say, take what you like and leave the rest. Like in Aurisarion, the mythology is these two worlds come together every I think 70,000 years that so close that it opens a gate and lets magic flow from one planet to the next. So with that, you have a setup for why magic exists here. And you have the history of another planet that I can make up as well as the history of Earth. You just kind of go from there.