Unless they make the original version of part III (Braniac, Kandor, etc) then I have no hope for this flick. The magic idea with a Faye Dunaway caught up in the downward spiral of Mommie Dearest just was too z-grade to work. I still mourn the waste of Peter O'Toole. The Salkinds were obsessed with making a cheap movie after Superman III failed to perform. Cannon only agreed to allow Reeve script control to get that financing they later squandered on everything BUT IV. The Salkinds fired Donner who actually was successful. After the flop of III, no way they give Reeve control. In fact, they made Supergirl to do away with Reeve in order to have a soft reboot and a cheaper production (their attention and capital was too invested in Santa Claus: The Movie). It didn't help that Reeve made it known he hated working with the Salkinds, was tired of being typecasted, and seriously wanted to move on.
You want a Superman/Supergirl movie? Keep Pryor from talking about how much he loved Superman II in an interview, the Salkinds never reach out to him, and we get the original part III as mentioned above. Of course it makes Supergirl a love interest, but it keeps the big budget, has a larger scale, and can't be worse than what we got (and I personally love III despite it's B grade quality).