You sort of have to be productive to write the Cosmere, because it's really complex. Did you have it planned in advance when you first started, did you really have a very, very clear idea of what you wanted, or was it just the structure?
So, for those who don't know, it has been referenced, my epic fantasies are connected behind the scenes with a lot of secret characters who are moving between the different stories. If you haven't read my books, don't get intimidated by that. It is mostly to be found if you dig for it, but not intended to be distracting from the main story of each book.
And it did start from the beginning, at least from the beginning of Elantris, which was actually the sixth book that I wrote. It wasn't there in the first few books, but by the time I wrote Elantris it was there. I can trace the idea to a couple of places. From a very young age, when I would read books, I can remember doing this for Anne McCaffrey, it was always very fun to me to imagine a character that was hiding behind the scenes in the story that she wrote that I had inserted, that the other characters didn't know this character's secret motive, and they would appear in the various books that I read. I would say, "Oh, that's him. Ooh, that's him in this other book," written by different authors. That is the origin of the character Hoid, most likely.
I can also point toward Isaac Asimov as an inspiration. In the late 80's, early 90's, when I was first becoming a big fan of fantasy and science fiction, I read Foundation, and then read the robot books, and then read his connecting the two of them together, which was one of those moments that broke my brain, and as I've read other people's works, I've found other authors who did similar things. Michael Moorcock is one, even the Marvel and DC comics did a lot of this. Famously, Stephen King did it with the Dark Tower books.
One theme I've noticed is that many of them feel like they decided to add this as a feature after having finished several books, and thought, "What a cool idea, I will connect them," and having seen them do this, and like it, I ask the question, "What if someone started from the get-go, from the first book, setting up a hidden epic behind the scenes?" Like most writers, I owe a great deal to those who came before and provided inspiration for the things that I do.