The ending

Deciding on the ending is important too. Do you want to finish using the same sequence as at the start? This could focus the audience's mind on the characters and how they came together in a fateful manner.

Do you want to have character 3 in a wheelchair - facing his life without hope of walking again? Perhaps using a monologue to tell the audience how he feels?

Do you want the families of each of the characters trying to come to terms with what has happened?

The final scene must have impact, and make clear the whole point that you intend for your drama.