So, it's a little of both. I did have the idea for Naruto and Sakura's growth and choosing one another even though they had lots of other options
before the manga ending. There was always going to be a handsome doctor to turn Sakura's eye, and there was always going to be a bit about Naruto's adventures and what goes into making him a good Hokage.
Those were the two points the story turned on:
1. What would happen if there was another person in the mix of Team 7 for Sakura to choose from? One that was in the middle of the spectrum between Naruto and Sasuke and combined their good parts? So Akira was highly intelligent and nice, he was skilled in Sakura's field and he recognized her as a brilliant in her own right. How would this change Sakura? And if she had other choices, would she still choose Naruto? What makes Naruto and Sakura soulmate material, when they only have shared experiences and not much else? Would she still choose Naruto over someone who was seems like the perfect match from ninjaMatch.com?
2. How would Naruto let go of his childhood ways and moved on to his adulthood goals, but doing it his own way as he has always said he would? What would his journey to Hokage include, what would he keep and what would he leave behind? And exactly how would he be the same goofball person but be someone who would make a good Hokage? And with this big shift in his life to maturity, would his childhood infatuation of Sakura stand the test of time? Would they still be a good match, or is there someone else out there better suited to his rambunctious personality?? 
So, those points were always there. I spent more time on Sakura because I felt like she had a more important story to tell as she was the one who was always the third wheel in the manga. She became a shinobi and understood Naruto and Sasuke's world, but they made no attempt to understand hers. So I wanted to tell that story. I figured I'd spend a little time on Naruto but not as much because he got a whole manga dedicated to his goals, and everyone already knows his story, motivation, etc.
But when Kishimoto imploded the manga, I realized there was a lot more about Naruto that needed to be told. And I really spent a lot of time thinking about why I was dissatisfied with the ending, what wasn't there, and what should've been. So as a result, Naruto's side of the story, in this fic, got a big injection. More characters, more plot, etc.
So it's a bit of both. All of my stories have already been written to completion (although in really, really rough draft form). I won't post it publicly unless I know how it's going to end. But after the ending of the manga, and after the letdown that followed when I wasn't sure if I could go on, I realized that I really needed to keep writing because I don't feel like the manga is finished. I feel like it just broke off at the end, and they glued a different ending on. So more than ever, I needed to finish all my fics so that I created the ending I didn't get to see in the manga. Kishimoto could screw up his work, but that didn't mean I had to screw up mine!!
Hope that answers your question, and I hope I didn't spoil anything!!