I would much prefer The Authority not play a big role in the Superman movie, or even appear. I don’t want to see an adaptation of “What's So Funny About Truth, Justice and the American Way?” (with the real Authority and not The Elite) as the first Superman film of Gunn’s DCU. he’s setting up a brand new Superman world and it would be stupid to go straight to a big moral conflict before the character is truly establish. It would be repeating the same mistakes of Batman v. Superman.
I know a lot of people want them to save Brainiac until later down the line but they need to kick this new DCU off with a massive bang. Audiences have seen so much already from the superhero genre and the DC brand is still pretty trashed. Some lower tier villain isn’t going to cut it. There’s also gonna be a ton of complaining from the Snyder bros pissed about the Cavill situation. Gunn needs something huge like finally bringing Brainiac to the big screen to drown out the angry static and put butts in the seats.
Since Gunn is such a big fan of Morrison, I wouldn’t be surprised if Superman first encounters a Brainiac similar to The Collector from the New 52 Action Comics. A more robotic being. It would also be a way for Clark to discover aspects of Krypton’s LEGACY. Then in later movies we find out that the real Brainiac (a more traditional version) is still out there and is an even bigger threat.