Possible, but unconfirmed, SPOILERS follow.
The rumor comes from the Latino Review, and claims that Superman’s mirror image enemy, Bizarro, will be introduced in Batman v. Superman, as a threat to the Justice League in future films. While this isn't too far of a stretch, we even considered it as a possibility, it'll be interesting if it actually happens.
Is Bizarro the much talked about “bigger threat” looming over Batman v. Superman: Dawn of Justice? That seems unlikely. Bizarro is great and all, but a clone of Superman would have to feel like a lesser threat than Doomsday, the monster that killed Superman (in the comics), though the Bizarro is connected to Doomsday's mythology by way of a Silver Age Superman story.