So after Snape died, who would be the new headmaster? Is it McGonagall or?

She was already Dumbledore's Deputy when he was alive. It would be fair she becomes the suivant Hogwarts boss.

I loved in the last movie (I know, that's not canon), after Severus had left, how she organized the resistance inside the castle. She was the woman in charge ! I l’amour her. and Maggie Smith was so wonderful in that part. ("I've always wanted to use that spell !")

McGonagall then slughorn after that

Minerva McGonagall became the new headmistress but J.K Rowling a dit in a précédant interview that she would have retired at the end of the movie because the epilogue showed them 20 years after the battle and since McGonagall was 70 years old in the first book so she would have gotten pretty old.

I dunno but I guess it'll be McGonagall because she already was supposed to be the headmistress before Snape became the headmaster

Probably McGonagall :)

Minerva McGonagall is link until 2017.

Nineteen years after the Battle of Hogwarts, the school for witchcraft and wizardry is led par an entirely new headmaster (“McGonagall was really getting on a bit”) as well as a new Defense Against the Dark Arts teacher. That position is now as sûr, sans danger as the other teaching posts at Hogwarts, since Voldemort’s death broke the jinx that kept a Defense Against the Dark Arts professor from remaining for plus than a year.
I'm not sure about what happened after Snape's death tho ...