@idcletmein123 *looks up from blank document* "...Plan?"
In all seriousness though, I don't really do much planning as far as story arcs go. I'm fairly aware of how my brain works. The moment I start railroading myself down a preset path, my enjoyable hobby becomes a tedious chore, and I'll very quickly get burnt out over it.
That's not to say I don't have ideas for things I want to happen, of course. I've got plenty of notes for fun worlds I could send them to, lots of snippets that are located too far ahead in the timeline to finalize or post, and a plethora of noncanon scenarios that are, to put it bluntly, fanfiction for my own fanfiction.
As for my inspiration? I've read a lot of Watcher Grian stories, but I found that very few really let him be comfortable in his own skin, usually using him as a target for angst. Nothing wrong with that! I just wanted to see a few where he's mentally stable, so I did it myself. And... My younger sister and I might have a tendency to make up stories about our favorite characters before bed (since it lets our brains wind down, unlike electronics), and an idea lodged itself in my brain for a couple days.
I figured, since it stuck around, I'd write it out! Then the one scene, built around the little compilation of different versions of HTTYD's nightfury leitmotif, became two. Then three. I was originally just going to have it be a way to set the scene for some fun shenanigans, without any real overarching plotline, but it grew on me. And when I was having so much fun writing, why should I not continue? I could write the exact things I wanted to see, and while there are definitely some scenes I find to be a bit "meh" when reading, I can confidently say that I've written something I love to read.