As much as I would love to see Ms. Marvel in this movie, I would be very disappointed if she was only added because people complained about gender imbalance. Put her in the movie because she's a cool character, not to fill a quota. Keep your affirmative action out of my movies!
As for Widow and Hawkeye being in the same team as Thor - The Ultimates did this best. Widow and Hawkeye were in the less 'public' team, doing the quiet jobs that needed a bit more subtlety than is possible if you send in the bloody God of Thunder or a guy in a heavily armed suit of mechanised armour.