Well if I'm not mistaken, the movie would somewhat detail the events of the first Metroid game so if they really wanted to stretch and make it into a movie:
1)Her origins, how she got her suit and powers and what not
2)Her training
3)Her becoming a Bounty hunter
4)The people getting her to help
5)Her adventures on the planet
I think parts 1-4 would make the bulk of the plot while part 5 would be the action.