I think I might write a Nyx/Reader/Gladio multi-chaptered fic. The story is basically the story I have for my FFXV OC, but I would be writing it in 2nd POV like my usual reader inserts.
The story is essentially Reader originating from a parallel world that is Versus XIII and then dying and waking up in the world that is XV. It would mostly be a story of Reader trying to adapt into this strange new world and then easing into the basic story of Kingsglaive and XV.
In terms of romance, it would essentially be Gladio developing feelings for Reader but Reader having feelings and entering a relationship with Nyx. But then Kingsglaive happens, and then Reader joins the party and slowly moves on and develops feelings for Gladio as the story progresses as it would in-game. (It would be really slow burn :P)