Do you ever dive into fan theories? And has it perhaps helped you to come up with a solution for some plot issue you have trouble figuring out?
Plot issues, no. Because, if I'm having trouble with a plot issue, that's usually during the outlining stage, and it happens well before fans are seeing things and can theorize on things. But, once in a while, in writing group I'll be doing something, and they'll start theorizing, and I'll be like, "Oh, that's a way better idea than what I had!" And that helps a ton; writing group can do that. The danger of writing group is, sometimes, they can take over the book. But as an outliner (as I am), that never happens to me. So if they are theorizing about something...
Really what it helps me is when people are theorizing along certain directions, it tells me what they're interested in, what they're thinking about, what they're expecting. And as a storyteller, one of my big goals is always to be in control of reader expectations, at least on the large scale, so that I know how people are gonna respond to what I'm writing. And I am creating and shaping that experience for people, and fan theories are really great for helping me understand where I need to put emphasis, what I've explained, what details are foreshadowed well enough.
I'm of the philosophy that most major things that happen in a series (like the ones I write) should be foreshadowed well enough that people are figuring out what's happening. This doesn't alarm me when people figure out early what's going to happen, because this entire series is about the journey. And I feel like if my signposts are correct, people are gonna have a general instinct.
That said, I always do like to add a few zingers that people aren't anticipating at all. I like those, when either they're the sort of zinger that surprises the cast; something happened in life that nobody's anticipating. And the fun is how people respond, rather than the actual surprise itself. Or the sort of surprising-yet-inevitable; the things that you aren't expecting until it happens, and then realize you should have expected it. So, I do like to throw those out now and then. Like, the little twist that happens with Adolin at the end of Words of Radiance; this is not something that I think people could have guessed, but it makes sense with his character. And so people were shocked, but not surprised, if that makes sense. And those sorts of twists, I really like to do.
But fan theories, I do read them when they pop up on Reddit. Mostly because people are asking me about them. And I find them very interesting. But they're more relevant in a "market research" sort of way than they are in a "figure out how to fix this problem" sort of way.