I have two thoughts on how we could do this. 

One is during the game: 
[i]
The gang is all alive, but instead of ending up taking the turn it did in the game, our Oc's (if you choose to use one) influence the fates of the characters differently, ending up twisting the choices and circumstances that would have been. 
Basically taking off from one point in the game and slowly shifting the story differently to unexpected turns and twists. [/i]

Or,

After the game where the ending is changed: 

[i]After the events that took place, the gods, realizing their mistakes and unreasonable prophecies upon those who made the necessary sacrifices, are given another chance to live.
Thus meaning Noctis lives to become a true King, and breaking the boarders so all of Eos is just one continent.
Demons (debatable) still are a regular occurrence and yet again humanity learns to live among them.
Ardyn is granted a second life and ends up living his days learning to live a different life he had ended up losing/ and abandoning long ago. Maybe he still retains the many powers he gained from his past, but he is no longer the seat of all corruption and Scourge.
Ignis regain his sight after Luna (who is also revived) heals them/ or perhaps regain them on his own/ or he never regains his sight.[/i]