They could always do what Wonder Woman and Captain America did, and have a major timeskip between movies. Wonder Woman's two films were set 70 years apart, and so were Cap's. For Superman, it'd be maybe 15 years. Set the first film around 2012 and the second one around 2027 (i.e. the present day, which it will be by then). That way, the sequel could introduce the Jon Kent Superboy.