I CANNOT picture Madonna as the innocent Maria. Perhaps, she could play the Baroness. Seriously, Maria's an innocent - Celine and Madonna are both too mature to play the role.
2ND - I can't see a remake of this working unless they adapted it. Example being the Preacher's Wife with Whitney Houston was a remake of Bishop's Wife. Same essential story, but set in current time. Then, there all he various versions of Christmas Carol - same underlying story with somewhat different details. For example, Vanessa Williams' Diva's Christmas where she played a big pop star who had alienated everyone around her until she was visited by Christmas ghosts of past present and future. Same ordeal as Ebenezer Scrooge, but different detail.
I could see Sound of Music set in an urban setting. Dad works all the time to get ahead. Kid's Mom left them for career in music, got involved with drugs and died. Kids are in day care and have care giver who is into music. yada yada.
Just can't see the noviate and the grief stricken widower remake.