Read that title again. Now let that sink in for a minute. Yes, I just said that. The Red Skull is secretly Thanos. The Mad Titan is not at all who you thought he was. He's totally NOT a purple-skinned alien from Jupiter's moon that can't get out of his chair because it is "not his time," he's secretly an ex-Nazi war criminal who is still bent, after 70 years, on world domination.
Let's examine the evidence. (I am aware that this is not an original fan theory, but I decided to give my views on it.)
Red Skull Is Sucked Into Space, He Doesn't Die
In the above photo, we do NOT see the Red Skull being atomized by the Tessaract, but instead, we in fact see him being sucked into outer space. And that's clearly not the Milky Way he's being sucked into there, so is it possible that nebular gases or radiation somehow interacted with the imperfect super-soldier serum to alter Red Skull's physiology in such a way as to turn him into Thanos?
Their facial structure is similar, and the fact that his skin has gone from red to purple may be a result of the serum's interaction with nebular gases as mentioned earlier. All of this makes sense if you start from the premise that Red Skull is secretly Thanos. But what if we were to reverse our starting point and say that Thanos is secretly the Red Skull and his apparent "death" in Captain America: The First Avenger was just when he decided he'd had enough of his charade and had attempted to capture an Infinity Stone, only to have his plan backfire catastrophically because his forces were constantly outwitted by "a simpleton with a shield"?
Think about it: Thanos wants all the Infinity Stones. He knows one is being kept on Earth in Norway. Knowing however that he cannot appear on Earth as a purple-skinned giant without arousing suspicion, he dons a clever disguise as Johann Schmidt and creates an entire persona for himself to hide his true nature and activities. Then, in the scene where he unmasks and reveals the Red Skull, people see what they want to see.
Because humans in the 1940s had little to no contact with aliens, their brains projected onto the alien form something that seemed reasonable based on given information. Their minds would actually be effectively tricking their eyes into seeing a red skull because that was the most comparable image to what they were actually seeing. Or, alternatively, Thanos may have realized that the humans would not accept his natural appearance and donned a mask over a mask. Therefore, it's not entirely unreasonable to say that the Red Skull never actually existed at all and his entire persona was secretly a coverup for the Mad Titan to obtain one of his coveted Infinity Stones.