This is a terrible idea. I may be in the minority, but this doesn't encourage me at all. Batman coming on the scene before Superman is a fully actualized character bodes ill for the movie. It screams more CGI action and limited character development. If Batman is cast wrong, even worse.
This idea needed to come after
the MOS sequel. Superman could really be overshadowed by the Batman character. I don't trust the MOS writers to balance anything in terms of story. It screams "gimmick!" Then again, I've never been someone clamoring for a justice league movie. It's all just a little too precious.