Here's how I predict it'll go:
1. Game starts 15 years ago. We learn about childhood Noctis, his special promise with Luna, and see Noctis's accident/near death experience where he breaks his arm and we first see his power.
2. Cut to present. Gameplay begins. Noctis and his buddies take the car so Noctis can meet up with Luna. On the way, Promoto makes a mistake and the car gets wrecked and they end up in Duscae. Around this time the shit goes down with Nifelheim and Lucis at the faux signing ceremony.
3. Once they learn Lucis has been overcome and the crystal has been taken, they hit the road, and the core events of the game begin.