Kara in a solo Superman film would be great. Keep the prequel comic canon or throw it out. I don't care, but get her involved and above all... KEEP HER AN ACTUAL "GIRL", none of this 27 year old woman going around called SUPERGIRL. Kara as a character doesn't really need to be a full fledged adult at all. This isn't a Robin or Speedy situation. In fact most of the powered "sidekick" characters could work. The suspension of disbelief for Kara or Kid Flash has a much lower bar to pass than say a teen aged Robin or Speedy. Super powers, bona fide SUPER POWERS is the great leveler.
It's believable that Kara can take on gigantic threats because she has the same base powers as Superman. She can be a 15 year old girl and it can work. Plus keeping her that age allows for something different rather than just having a sexualized female version of the hero you can get into issues of mentoring, learning, growing up, emotional maturity ect... I mean, I always thought that Supergirl is a great character to showcase Superman as a parental figure, where as in the comics for a long time now Supergirl seems to exist for the purpose of cheesecake or the like.
So I say bring on Kara in the DCEU, but let's get something different that what is going to be on CBS, okay? Is that so much to ask for?