I never thought about doing a sequel when I was actually writing 'The Magicians. ' I only ever considered it a standalone.
What makes a great standalone piece of hardware is not the same thing as what makes a great networking device. One can work as an essentially closed system. The other is absolutely dependent on its openness.
I do open endings on purpose. I expect a lot from my readers. I want them to do much of the work, because I believe that the story is built by the reader, not by the writer. I like having an open ending to a standalone fantasy, because it allows a continuing story to be written in the hearts of the readers.