ok first off i would like to say its not onyx, onyx is covered in lush forests and the planet seen in the trailer and the end of halo 3 is bare and metallic so what i think it is either a shield world(unlikely) or to make the storyline good, a contaiment planet for the flood. Thats what i think because the floods not gone, its just slowed so yes i think flood is going to be the main enemy and sentinals will be a secondary threat. thats what i think