After watching the Batman v Superman: Dawn of Justice trailer several times over, we have come to the conclusion that the possible villain of the standalone Batman movie could be the Red Hood.
How did we manage to come to this crazy conclusion? Well, from the trailer, we get to see Robin's burnt costume with the words, "Ha Ha Ha, Joke's On You Batman." From the comics, the Joker beat Robin to death with a crowbar, then set Robin's body ablaze.
This particular Robin is not Dick Grayson, but none other than Jason Todd. He later returned as the Red Hood due to Ra's al Ghul bringing him back to life by way of the Lazarus Pit.
In the Batman v Superman: Dawn of Justice trailer, Bruce Wayne was sent a letter: a cutout of a newspaper article talking about the many people who died in Metropolis in the battle between the Man of Steel and General Zod.
On the newspaper cutout, someone wrote the words in red: "You Let Your Family Die." Apparently, this person knows the identity of the Batman as Bruce Wayne. Not a lot of people know this secret, and not many have a reason to send Bruce a letter about letting his family die.
We believe the sender of the letter is Jason Todd. The reason for this is because Bruce Wayne considered him as family, and Todd viewed Bruce in the same manner. In the comics, he blamed Bruce for what happened to him and used it to try and get into his head.
We suspect Batman v Superman: Dawn of Justice will introduce moviegoers to the origins of Jason Todd as Robin, along with how he died. We believe this movie is also a build-up to the Batman standalone film where Jason Todd could be the Red Hood.
Since fans already know a lot about his origins, there's a chance the character could be revealed in Batman v Superman: Dawn of Justice, and from there the writers would gradually set him up for a new Batman movie.
There's no reason to bring the introduction over to the standalone Batman movie because many fans would have already known who the Red Hood really is from Dawn of Justice. The new movie might just kickstart the film with his identity already out and build from there.