I'd love to see Kara, but I don't think Singer will do her. He really doesn't do female characters well, as evidenced in his other films.
Furthermore, I think bringing in Jason as Singer (foolishly did, IMO) really diminishes the chance to utilize Kara/Supergirl in a Superman film. I really don't think we'll ever see her in a Singer influenced Superman film.
It's really very sad, because I feel the best interpretation of her occurred when she was used in STAS, and then her character evolved and grew into JLU. She also gives Superman a viable backup to deal with menaces that are too big to deal with alone.