My idea was to put together something that looks a little more modern, is comfortable and practical, and might appeal to an actual teenager. I went with a sort of motorcycle-leather inspired look, swapping out a red jacket for the usual cape, shiny blue pants for the usual skirt, red sneakers instead of boots (which would be the one part that's not motorcycle-inspired), and a tank top with the iconic symbol on it to tie the whole thing together into a recognizeable whole:
Close-up on the top, showing Supergirl's neat little ribbon choker (a girl's gotta have some style):
Here you can see the shoes a little better:
The view from the back shows the hairstyle I chose. I wanted her hair to be pulled back from her face, so it wouldn't get in the way while she's kicking villain butt:
And here's a pic with the jacket off, so you can see her tank top a little better:
If it were me flying around and fighting crime, I'd probably want a higher-necked top, and my belly covered - but Supergirl is a teenager (and I'm not positive that I ever was - I'm definitely not one now), and I thought a tiny sacrifice of practicality for trendiness made sense for the character.
Anyway, that's my Supergirl OoaK! I may take some more photos tomorrow, if I get a chance, so that I can not use the flash, and get truer colors.