My personal opinion is I'd prefer they introduce her like they did in Superman Unboubd where she's already here as Supergirl. The film could be set anytime after Justice League. Then you explore her origins in a prequel film, personally I'd rather they avoided that Jeff Loeb storyline from the comics. I think her origin should just be her and Clark. Darkseid should be saved to get taken down by the Justice League in a film where it's just the JL vs Darkseid.
Well, that's what I was saying. Break SfK into two movies, the first as a Supergirl origin and the latter half as a JL sequel dealing with Darkseid. That's
if he doesn't die in the first JL movie... Or some other big bad.
Basically, I like how she was introduced in SfK. Both it and the New 52 intro were both done very well. I like in both how she immediately starts causing mayhem, however unintentional, although when she meets Kal in both, the reaction in each iteration is completely different.
Although, I don't know if they can still pull off the cousin-part? In some flashback scene they would have to go back to before Krypton's destruction and introduce her parents as related to Jor-El. They could use the New 52 story line for that, as describing Jor and Zor as being estranged, with the younger Zor-El copying his brothers work.
But however they do it, I want a big screen original for Kara rather than having her just be there, and then do
another prequel movie. It worked good for WW, but I wouldn't want to reuse the formula.