I always liked Imogen Poots for Supergirl. Mia Wasikowska was my other pick. Personally I wouldn’t want Kara to feel American, and so I think casting a Brit, Aussie or whoever with a somewhat unconvincing American accent is more appropriate.
I also think these two actresses would nail a more quiet or more pensive take on the character. After all she’s stranded on a different planet and just found out everyone she knows is dead.
I think a Supergirl film could be an interesting film to deal with the pressures of being a teenager, trying to fit in, etc. What better character than an alien teenage girl with superhearing to deal with these themes. Id like to see the film take cues from Eighth Grade, something that really puts us in the mind of a girl in situations where she feels an immense amount of pressure. Shades of Carrie could be interesting too (obviously with less disastrous results).
It could be an interesting film to teach girls that no matter who you are, even if you’re Supergirl, the kids in class may think you’re ugly, or have terrible fashion sense, or stupid, but that’s okay, because maybe you should just focus on your strengths (like superstrength!), what you can actually control, and what kind of woman you’re going to be in the real world once school is over.