Who Is the Avengers Villain?

So it's official.  The Avengers have started filming, which is a terrifying concept.  The project was announced almost three years ago, and it's always been theoretical, something so impossible to create you wondered if it would ever become a reality.  And it's filming.  Please, please don't mess this up Marvel...

The funny thing?  We've heard one casting announcement since SDCC last summer.  To be fair, most of the roster was set in stone and already in place.  That was kind of the point.  But the only new recruit outside of the team itself has been Cobie Smulders as Maria Hill.  The movie doesn't really need any more of a supporting cast considering it has, what, 7 main characters?  But there is one casting choice we have yet to hear.  The villain.

Look at them without at a villain!  Standing around, trying not to look bored.  It's embarrassing.

Let's assume that casting is done, and that the movie is in fact shooting.  This means the villain is someone who already exists in the Marvel Cinematic Universe or will be done with CGI of some kind.  This rules out some of the more human villains like Baron Zemo, or even guys like Thanos who need actors underneath all that makeup  Who does that leave?  One option is the Loki and the Hulk tag team.  This has been one of those rumors that has been thrown out and rumored about for quite a while, and is also the original story from the comics.  I'm not a huge proponent of this theory, since I think you can't really justify the Hulk as an ongoing threat or force of evil, even with mind control through Loki.  The Hulk will be at odds with the rest of the team, but I still don't like him as the biggest threat for our heroes.  And you can't have Loki be "the" villain, because he's ultimately Thor's arch nemesis and a threat we know the God of Thunder can handle on his own.

The other possibility is the Skrulls (or whatever they call them in the movies).  Considering you don't need to cast any specific Skrull characters and just need a bunch of the aliens themselves, this is still a viable option.  Then again, we need at least some of these damn dirty aliens to talk, and they can't all be part of some faceless alien horde.  So I would have expected there to be at least one Skrull cast, but I could be wrong.  We've heard more than a few rumors about the Skrulls as villains, so this could still be possible.
 

He's just as confused as I am.

I'm really seeing these as the two biggest options at this point.  Between news and rumors, as well as simply plausibility, they make the most sense.  Unless Marvel is being really, really sneaky that is.  And I wouldn't hold it past them.