I usually prefer also playing my characters in universes with some story as well so this works out for me. I can definitely play both if you find it difficult to decide or would prefer me to play both. 
I'm confident in getting their characters down.
Alright, sounds good. 

Ah man, I researched his character and others like crazy once I finished XV. I just had to know more so I invested a lot of my time exploring lore and theories. 

Just the typical growing romance. I usually prefer those since it gives me time in fleshing out my characters history and important factors with the Canon character. Plus I always enjoyed the slow growth and reactions.
As long as you keep it as close to the character as you possible are able, I'm pretty satisfied.

I kinda grow the plot as I go, but I definitely make discussions before hand.
Not to say I don't have a plot for my Oc as well that's basically an original story, but just that I need to flesh it out with Ardyn's and that's where discussions come along. 

Anything your particularly asking about what I had in mind for Ardyn x Oc? Incase I totally missed the point.