I hate those fan theories! They're usually lame and written by someone who doesn't know the source material very well. Having said that, I offer you a fan theory...but don't call it that, ok? Let's call it "speculation based on what we already know". Could any of this be considered spoilers? Maybe, but probably not. It's all common knowledge or my own theory based on known facts. The subject of the speculation will be the plot of the upcoming Batman/Superman film, "Dawn of Justice".
Although elements of other story lines look to be blended in, this film seems to be shaping up as a clever use of the Frank Miller classic "The Dark Knight Returns" (TDKR) as a starting point rather than an epilogue. The recently released second trailer contains many clues as well as a few outright revelations. Recent statements by the principals involved in the film also have provided some insight. It seems Bruce Wayne had a building destroyed in the battle between Superman and Zod. Wayne is shown consoling a little girl in the aftermath. The trailer then makes it appear that Bruce decides at some point after this that it is time for the Batman to return.
Why do I say "return"? First, DC and Zack Snyder have said from the start that Ben Affleck's Caped Crusader would be a 40-something year old Batman, not Affleck trying to play younger. Second: there's a flash in the trailer of what appears to be an old estate in ruins (Wayne Manor...?) and then a shot of Affleck looking at the Bat suit with determination in his eyes. There's a glimpse of a Robin suit, that looks to be maybe burned and spray-painted with the words "ha ha, joke's on you Batman". Finally, Batman is seen on the roof of the Gotham Police Department building, pulling a tarp off of the Bat signal and turning it on.
Jared Leto is rumored to have a cameo in BvS. The spray-paint on the Robin suit is an obvious reference to the Joker, after all, so it's logical to assume he may appear, if even briefly. Leto's version of the Joker seems very similar in appearance to the one depicted by Miller in TDKR. As many already know, in Batman comics, the Joker killed the second person to wear the Robin suit, Jason Todd. Todd's death is referred to in TDKR as well. Are they planning to use the murder of Jason Todd as a way to explain why Bruce retired to begin with?
Another question that has been asked (will we be seeing a Robin in this film?) may also be answered by this theory. When news first broke of this DC epic being in production, there were rumors flying daily about the cast. One of those rumors was that Robin was indeed going to be in the film, but it would be the female Carrie Kelly version from TDKR. Internet Movie Database has Jena Malone listed as "rumored" to be in the movie, and those rumors usually involve her playing Carrie. Christina Wren, however, has been confirmed as a character named "Carrie Farris". It's doubtful DC would have two "Carries" in the film, so could this be a renaming of the character, or possibly a fake name to keep it secret?
Another element seemingly lifted straight from the pages of TDKR is the suit used by Bruce for his showdown with Superman. Fans of Miller's story will recognize the armor the minute they see it. The wealth of possible reveals from this second trailer continue, with glimpses of Lex Luthor and some nice footage of Gal Gadot's Wonder Woman. An underwater shot shows a scene that will most likely be involved in the events that bring Aqua Man (Jason Mamoa) into the story. I'll keep this easily digestible by saving those discussions for another time.