The casting website, acting-auditions.org, has made a post inform Michigan residents how they can become an extra in in Man of Steel 2. The various information that they provide about the film's potential titles, "Batman vs. Superman" and "Superman vs. Batman" come from a statement that David Goyer made at San Diego Comic-Con (which is mentioned on acting-auditions.org's post). As for plot, they simply state what we already know, "will feature Superman and Batman meeting for the first time in a cinematic format."
As for Detroit being Gotham in the new film, well the site explains, "Detroit is being touted as the location that will stand-in for Gotham City." Once again, just regurgitating information that we've already heard from various sites.
The owner of Acting-Auditions.org stated in a previous interview "information [is] sent to me directly from casting directors and production companies" in addition to doing his own research.