Much like Ryan Reynolds abandoned Green Lantern for Deadpool, another Marvel movie star has jumped ship to DC. Former Green Goblin Willem Dafoe has joined the Justice League film as an unspecified "good guy", according to the Hollywood Reporter. And we have a couple of ideas about who he might be playing. Assuming the ultimate villain of the two JL movies is Darkseid, someone is going to need to explain to the heroes (and mass audiences) who this person is, what's his deal and why he's so terrifying. And you can't just pull Johnny Knoxville out of retirement for this key exposition; it needs to be someone with charisma who can sell Darkseid's shtick and make it sound menacing — and Dafoe fits this bill perfectly.
But what "good guy" would possibly know this information and could tell the earthbound heroes? We have two potential characters:
1) The Highfather
The "good" equivalent to Darkseid is an obvious choice to tell Earth and its protectors Darkseid is gunning for them. I believe that Snyder and the WB will want to get into the New Gods as little as possible — it's Jack Kirby insanity that mass audiences couldn't begin to handle — but keeping it to Highfather and maybe a minion to transport the League to/from New Genesis to get a pile of exposition would keep it simple, and the rest of the New Gods wouldn't need to be seen or mentioned. Remember, even DC's comics have positioned Darkseid solely as a Justice League villain for quite some time now.
2) Superman's Kryptonian spaceship/Fortress of Solitude operating system AI
But if WB wants to avoid everything New Gods, then the records of Krypton are the next obvious choice to have info on Darkseid. Sure, the Kryptonian ship's computers have been significantly un-Willem Dafoe-like so far, but it also hasn't need to provide such fundamental plot information before. Hiring an actor to personify it is a natural choice. Also, this gives Snyder and WB licence to corrupt the AI later, and have Willem Dafoe as Brainiac (which he would be perfect for) as a sort of anti-Vision. Hey, just because Dafoe's playing a "good guy" now doesn't mean he'll always stay good.
(For the record, an even more obvious entity to give DC heroes information about extraterrestrial threats is usually the Green Lantern's Guardians, but WB impressively made Green Lantern so radioactive that he's not even going to be in the Justice League, so that's a no-go.)
These characters would also both explain why WB/DC is bothering to keep the specifics of this casting news a secret. Dafoe's got to be playing someone notable and interesting, but it's not like they made a big deal of hiring J.K. Simmons as Commissioner Gordon.
But these are only our theories, and surely you have your own — including some better than ours! Leave 'em in the comments!