(Warning - the following contains speculation relating to both the plot of Captain America: Civil War, and the Marvel Cinematic Universe beyond it. As such, theoretical SPOILERS lie below, so proceed with whatever level of caution your innate sense of SPOILER-aversion suggests is wise...)
Now, of all the geekily intriguing things hiding in the most recent Captain America: Civil War trailer (Ant-Man arrows! Armless mistakes! Underoos!) one in particular stood out to many fans as being cut of the most intriguing (superhero self-sewed!) cloth.
Specifically, the moment in which we caught a glimpse of a mysterious yellow tube - the contents of which remain a mystery. Y'know, the one you can spot at around the 01:42 mark below...
The big question now, then?
Just Who The Heck Is In That Tube?
Well, as of right now, the only thing we know for sure is that it isn't Bucky, despite Cap's erstwhile sidekick being the most obvious choice. After all, as Kory Glover recently pointed out on this very site, Bucky can quite clearly be spotted just behind Iron Man as he sucker punches Cap:
With Bucky out of the running, then, let's take a look at five alternative candidates...
5. It Could Be A Villain Of Some Sort
With the trailer's earlier introduction of The Raft, Marvel's premier floating prison, it's not impossible that we could be seeing some sort of major Marvel villain make their first (or a repeat) appearance. Baron Zemo, Crossbones, Emil "Abomination" Blonsky, Aldrich Killian and even a somehow reborn Red Skull (perhaps in the body of a loyal Zemo?) would all make for intriguing returns to the MCU in one sense or another — and could provide our heroes with yet another reason to squabble.
4. It Could Be One Of The Avengers
Now, admittedly, this one seems a little unlikely given the (blurry) appearance of the yellow tube-dweller, but it's not entirely impossible that we're seeing an imprisoned Avenger of some sort. Would Tony stoop so low as to allow his Science Bro Bruce Banner to be imprisoned on The Raft? Or, alternatively, could it be someone even more unexpected? After all...
3. It Could Be Quicksilver
Who, as many fans have pointed out, may only be mostly dead. Could the speedster replace Clone Thor from the original comic book storyline as 'hero press-ganged into service by Iron Man'? It would certainly provide one hell of a story arc for Scarlet Witch — and mean we wouldn't have lost one of Marvel's most interesting heroes for good.
2. It Could Be Adam Warlock
I mean, it (almost certainly) isn't, but it would honestly be remiss of me to discuss a theoretically appearing figure in a mysterious tube without mentioning Adam "Cocoon-fan" Warlock. An alternative cosmic option would be the Kree Captain "Mar-Vell" Marvel — though whether Marvel will ultimately end up bring him to the Marvel Cinematic Universe remains to be seen, despite his key role in Captain "Carol Danvers" Marvel's origin story.
If you're wondering why, just look at how many times 'Marvel' crops up in that last sentence.
Finally, then (and perhaps most likely, and thus theoretically SPOILER-ifically)?
1. It Could Be A Clone
But, y'know, not a Spider-Man clone (thankfully).
Instead, it's entirely possible that who we're seeing in that tank is, in fact, Bucky Barnes. Or, at least, his clone. With Captain America comic books having long played with the idea of Cap clones, and the comic book Civil War having done much the same, Captain America: Civil War would seem about as good a place as any for the MCU to introduce cloning to the party. Could we be set to see a Hydra-created Bucky make his debut — thus perhaps getting him off the hook for whatever he's being chased down for in the movie coughWakandanAssassinationcoughcough?