One can hardly argue of the appeal of a newly teamwork focused Batman repairing his relationships to characters like Nightwing and Batgirl, and then teaming up to fight the who’s who of Bat-villains.
Plus, with Nightwing, Batgirl (a potential Joss Whedon joint), and technically Gotham City Sirens in active development, all of these characters will be around for whenever the film hits theaters.
The relationship between Batman and his costumed allies in Gotham has never been seen satisfyingly on-screen before, meaning it would help to immediately set the Batman solo film apart from every other Batman movie.
While this rumor is very possibly fake, the addition of the Bat-Family would be a logical and smart move for Warner Bros, and crew to make.