The Theory: Despite numerous claims to the contrary, the Tom Hardy-starring Venom movie will in fact be a secret addition to the Marvel Cinematic Universe.
Why It Can't Be True: Marvel Studios has been very clear that any peripheral Sony movies set in the Spider-Man universe will be only tangentially-linked to the Marvel Cinematic Universe, presumably due to Marvel Studios' lack of quality control and oversight regarding those projects.
Plus, considering the headache-inducing nature of co-productions between movie studios, it's hard to imagine the two companies ever coming to terms agreeable enough where Tom Hardy's Venom can be slotted into the MCU in the future.
But..What If?: We all assumed Sony would never loan Spidey out to the MCU, so crazier things have most certainly happened, and even if the Venom movie ends up sucking, Hardy would surely be a major asset to the MCU.
Plus, if the Venom movie is even remotely decent, this version of the character could easily be rolled into the MCU, where Marvel Studios would have full control of how he's used, while Sony simply takes a cut of the box office and keeps their hands off.
The main point here is that if Spidey can join the MCU, it's entirely possible that Venom ends with a surprise cameo or nod towards him being an active participant in the franchise too.
Which totally insane 2018 movie fan theories do you most want to be true? Shout them out in the comments!