If you'd like to reacquaint yourself with my earlier rambling:
What if he's the Red Skull??
The case for:
- At the end of CAPTAIN AMERICA, I wanted to believe that the Cosmic Cube, aka Tesseract (when did it get that WRINKLE IN TIME name? - that really pushes its definition as a wormhole-opener over reality-bender), somehow assimilated the Skull's essence, maybe against his will, or maybe following a loose interpretation of the Skull's unspoken/subconscious wish - "ultimate power" or "superior being?" But, what we *see* -seventy some years in the past- is the first manifestation of the Cube's power as a portal. A rip in spacetime opens above the Cube and we see a different part of the cosmos thru it, and as the Cube seems to amp up in power in the Skull's hand, it "beams" him to some point on the other side of the portal. So, who do we know who present-day-ly lives over there on the "other side of the galaxy" (Hawkeye's words)? Why, it's Thanos, and his Chitauri cronies. So, I put forth that the Skull arrived in Chitauri space via Tesseract, perhaps driven mad(der) and transformed and perhaps supercharged by it, fought/negotiated his way up the Chitauri food chain, and became their Death-worshiping Main Man. To make it a bit more interesting, I *want* to add that the Tesseract also displaced him in time, sending him a couple centuries into the past, which could make him ancient, having lived many human lifetimes Out There, managing Tesseract-invasion campaigns (assuming there are multiple Cubes throughout the universe, and the Thanos and the Chitauri are "tuned" into their energy) for hundreds of years, waiting/hoping for Earth's to one day be activated again. (In the Marvel comics universe, multiple cubes have been created, and some of them have evolved into sentient beings.)
- This retcon of Thanos's origin would avoid having to explain the long-lived and twisted comic book history of the character, which at the very least would involve creating/introducing the Eternals of Titan.
Oh! Also, it would ground Thanos in a character we've already met. Also also, while Thanos could be all CG, it would be great to have Hugo Weaving back, no?
a. Crazy thought of a crazy thought: Would this open up the possibility for Captain America to don the Nega-Bands (I kind of not-love the idea of turning Odin's vault into a candy shop, but has anyone seen them there? There's always Project Pegasus, too, right?) and become Captain Marvel to fight him?
b. Would the movie tackle the idea of an actual avatar of Death? An aspect that can be seen/detected by more than just Thanos? Or does she only appear to him, leaving her real-ness ambiguous. Hrmm... Maybe it's actually Asgard's Hela! She could be in a legit worshipful relationship, or manipulating the Mad Titan. Hrmm... We could see Hela appear in the shadows of solo hero movies for a while to set her up, maybe even flashback to scenes with her from movies we've already seen...? But, how often can we go to the Asgardian well, right...? Hrmmm...
- Skull-Thanos (or is ThanoSkull better?) would have a familiarity and appreciation of Loki's pedigree. As a true believer in Asgard and Asgardian magic-science, it would make sense that he would recognize Loki's value, as an agent and as a pawn, and perhaps use his intel to learn how to reach and ultimately sack the Shining City for its power and vault of cosmic weaponry, including his beloved Infinity Gauntlet, which we see in THOR (next to the Evil Eye?). Hrmmm... Maybe when we see Loki manipulating Solweig at the end of THOR, he's already in Thanos's thrall, sent to ensure that the Tesseract gets activated on Earth.
- If he was the Pokemon-evolved form of the Skull, wouldn't Thanos want to see to an invasion of Earth personally? Or rather, Titanically? But, y'know, it's been (at least) 70 years, so maybe the Skull's learned a bit about patience, and, assuming he's managed invasions of other Tesseract-tech-developed worlds, maybe he's made it his policy to only show up once the planet is secured by his minions, a la Vader on the Blockade Runner at the start of A NEW HOPE. He also has absolutely no reason to believe that his super-soldier nemesis would still be around, so there's no personal stake in it.
- An explanation as to the transformation from Skull to Thanos. Not all that tough if you ask me. Schmidt had Erskine's proto-super-soldier serum in his system. We saw that "Vita-Rays" activated the final serum for Steve Rogers - who knows what direct exposure to Tesseract radiation (which includes the gamma spectrum, apparently) would do to the Skull, right? And again, if we go by what we *see* at the end of CAP, the Cube basically takes the Skull apart, bit-by-bit, and them modems him across the galaxy. Who knows how it might have tweaked him in transit. And again again - the Marvel movie universe has not explored or even acknowledged this aspect of Cube-ness, but - if reality-bending *is* in the Tesseract's wheelhouse, well, if the Skull wished to become the "superior man" capable of wielding the Cube as its master, might the Cube not transform him into a Thanos?
- Would Loki willingly serve a transformed human? Certainly not without some persuasion, but maybe he doesn't see or know Thanos's origin? Or, having seen ThanoSkull's great Death-worshiping works (which we have yet to see), he has been persuaded.
Keep on keepin on~