What's a piece of fanon that you suspect the writers believe? OR, What's a piece of fanon that has been recently (ish) canonized?

For example, the Federation's founding members (Tellarites, Andorians, Vulcans, Humans) were the subject of fan theories and "fanon" for many years before the ENT writers made it official. One of the interesting (and fun) aspects of this recent wave of series has been seeing the writers increasingly add nods to fan theories and pieces of fanon lore over the years. What are some good examples of this?

And relatedly: what's a fan theory, or piece of fanon, that you suspect the current writers believe, even if they haven't explicitly stated it on-screen?