Harry Potter will never wane in popularity as long as the community continues to engage with it at the level they do. Even aside from JK Rowling's attempts to capitalise on the fanbase by releasing the Fantastic Beasts movies or expanding the Wizarding World's lore through Pottermore and her social media... exploits, the fans are a perpetual machine under their own power as well.
Even more than a decade after the release of the final book in the series, new theories, new interpretations and new appreciations of the text continue to pop up. We are such an engaged fanbase who make returning to the texts and the films a frequent event that it's possible to read something new from intelligent, knowledgable fans almost daily.
And sometimes, crucially, fans come up with better ideas than the original source material to explain missing story details or to further expand key backstories. So much so, in fact, that you sometimes wish the keys to the Wizarding World were in someone else's hands...
12. Voldemort Is A Combination Of Tom Riddle And Salazar Slytherin
The idea of Tom Riddle starting out relatively "normal" - at least according to Horace Slughorn's assessment that "if a monster existed, it was buried deep within" - and then transforming into the greatest Dark Wizard of all time (hat-tip to Grindelwald) never quite sat right. Sure, he went through a tough ordeal and there is a lot of trauma in his family's past, but the jigsaw pieces never quite fit together - particularly when you consider that a half-blood wizard would be The Wizarding World's chief exponent for blood purity and magical might.
More compelling is the theory that Voldemort is a construct of two parts - Tom Riddle and the spirit of Salazar Slytherin. Harry Potter himself and Professor Quirrell both proved that two souls can share a body, after all.
The Hogwarts founder had infamously sought immortality and we know that the magic for Horcruxes already existed when Riddle came across it. The fact that it was forbidden suggests a dark history of practical application, but also Slytherin having sought the knowledge to use himself in order to preserve some of his soul would also explain why the books were kept in a school library. Otherwise, you have to accept that Hogwarts' teachers are all just REALLY irresponsible.
Even more pertinently, he rather symbolically stowed a basilisk inside the Chamber Of Secrets as his legacy that even more symbolically emerged from his literal face when awoken. What if Slytherin left a piece of himself in the creature too and it laid in wait for a parseltongue to lure into his Chamber in order to fuse himself with the child and be reborn?
That would atone for Riddle's dramatic change while he was at Hogwarts and Slughorn's uncharacteristic idiocy in believing him to be good enough for his Slug Club. Not only that, but it could also go some way to explaining a little better why Voldemort has such notably serpentine features rather than it simply being a matter of "well, he's evil and snakes are creepy, aren't they?"