I'm 95% sane on this, but I 5% believe that William Shakespeare of Stratford-upon-Avon didn't write the plays. A lot of mainstream and not-so-mainstream people now and throughout history have debated the authorship question and back different candidates (the "Oxfordians" are by far the most vocal/insane), but my favorite candidate is definitely Christopher Marlowe. This apparently makes me a "Marlovian." To be Shakespeare, Marlowe would have had to have faked his death. He was a famous playwright, likely government spy, and supposedly died after being stabbed in the eye during a bar fight.
I fell down the Shakespeare authorship rabbit hole and ended up taking this MOOC on Coursera. It was really fun! But then I discovered on my own that Stratford Will actually had a son named "Hamnet" (it may have been mentioned on the MOOC but I didn't hear it). I was like seriously, I sat through hours of circumstantial evidence that he didn't write the plays but you failed to mention his son was named HAMNET? That's when I went from "10% not the guy" to "5% not the guy." But as far as conspiracy theories go, this one is fun and pretty innocent.