As I've said before, Emily Blunt is the best choice
- Played similar roles before in terms of action and attitude
- Award-winning actor (not academy yet, but she's young)
- Star Power, second billing on lots of big successful films
- Good relationship with the MCU (second choice for Widow)
If your response is she 'doesn't do it for you' or she looks hotter with dark hair, then you're missing the point. She's not a love interest, she's the main character. Chris Evans may not do it for you either, but would that be why you wouldn't cast him as Captain America. Jennifer Lawrence doesn't really do it for me either, but Hunger Games is my ish. There are lots of other great actresses who could play the part well, but Blunt is the best positioned to career wise, from what I've seen.
There is this thing about 'I don't buy her in action scenes.' But that would require an at length conversation about what determines whether you "buy" a woman in action scenes or not. Do you really want to have that convo? I do... but do you?