She's a good actress, young and pretty. She'd definitely LOOK the part. But the only thing I've ever seen her in is GoT, where she's always played the meek girly-girl. I don't know if she can play a tough as nails warrior woman, which is exactly what Teela is.
That's why I was thinking the wildling girl. She's also young and beautiful, granted a bit older than Sophie Turner. But she actually has convincingly played a tough as nails warrior woman, which makes me think she might be better suited for the role.