I wrote a moveset for Lyn many years ago--and looking back, it was total garbage. That said, I think I'll try mine hand at another one.
Lyn WOULD be unique among the Fire Emblem characters in that she would wield a bow for at least her neutral special. They could also take inspiration from her ninja incarnation in Fire Emblem Heroes. Basically, her playstyle could have a mix between sword fighting, archery, and debuff-inducing daggers. Mine favourite thing about Fire Emblem Fates was definitely the new debuff mechanic they introduced. So this would be a great way to represent this gimmick in Smash. In one moveset I'm currently working on, her down-special has her summon copies of herself--in reference to her critical animation in Fire Emblem: Blazing Sword.
At this point, I'm not expecting another Fire Emblem character period. Especially given the silly backlash caused by Byleth's reveal. That said, if a new Smash game ever did get made, I could see Lyn getting added. I think a Smash game with a smaller roster (around Melee's size) that focuses more on balancing would be a refreshing addition. Any cut fighter could return as an Assist Trophy.
Speaking of Fire Emblem Fates, this song sounds VERY similar to the final map theme in Conquest. I wonder if the composer took inspiration from it. Wouldn't surprise me, since the lyrics fit the Conquest plotline perfectly.