Hmm yes, one or more Lanterns seems plausible. Early promotional material for the movie said "unite the seven", and now that more recent teasers had the words "unite the league" in them I think it's become clear that it wasn't referring to the seven seas and the Aquaman movie. So there's a seventh member missing that should show up in the movie. Man, that would be fun.
Eradicator or Cyborg Supes would be fun, too though. And the thing is they'd be mistaken for a hero at first, and they could also announce themselves despite the real Clark still being "dead". What about the two impacts visualized with that Jurassic Park effect? Some means of transportation for the Lanterns? Or Supergirl and one of the fake Supermen landing?
Speaking of Supergirl, that really seems to be a hologram of her when Steppenwolf says "no kryptonian", right? The leg seems female, there are golden lines on the blue parts of the suit and the cape seems shorter than Clark's.. plus Snyder planted that empty sleeping capsule with no skeleton in it in Man of Steel without having an actual Man of Steel sequel in mind, as far as we know. I think there's a big chance of her showing up.
Man, I'm excited for this.