Sad things only get sadder
During the first year after the stop of Meteor, a disease began and spread called Star Scar Syndrome. Nearly the entire population were killed, including the close friends of Cloud.
A voice of a small girl says in the trailer: "That day, the power was used. Therefore, the star now lives. The humans have become the star, their souls among the star. And I had thought it would all be over..." That voice, from my speculation, is Marlene.
Tifa died, as did Barret and everyone else. Cloud's motorcycle is known as "Fenrir".
After the silver haired man appears, Cloud is standing before a grave and says, "I won't be forgiven, but I want to be forgiven." A sword in the ground, usually interprets that the wielder of that sword died where it is stabbed. This sword was stabbed in the ground at Junon Harbor.
Cloud fights alone, because the world denied anyone else the right to live.
Prepare for the ultimate spoiler - The person in the wheelchair that Mystery Man walks up to, (it's been confirmed that it is highly possible that the Mystery Man is a reincarnated Sephiroth) is none other than AERIS. The fact that Mystery Man kneels down to grab her hand interprets two ideas: Sephiroth still loves her and regrets killing her, or Sephiroth is no longer evil and is trying to bring her back to life so she can reverse the effect of Holy.
What we pretty much know: The Planet is angry at humans for the evil acts they have commited two years before, experimenting on themselves creating dangerous fighters like Sephiroth who aspire to destroy the planet, and characters like Cloud who don't care about the planet.