15ish years ago I had a wet cold marvelous afternoon, despite a massive migraine, being a character in a live action role play. That choreographed plot led to me writing a story about it, then adding my friends characters, then switching to another online RP and making up more characters and having adventures beyond that.
I'm into the 4th book, editing the 1st and will do the same for 2 & 3 and have only my original characters and situations in the series.
It all started with 'what if there's this woman with scales on her left side' then there's how that happened, her family, her upbringing, her friends, her enemies, her loves and hates, what's happening in the larger world, what ties things together, and on and on.
Everyone has a past, a present and a future. No matter the genre here's always more to write about the person, the climate, culture, the civilization, the cuisine. Stories end at some point but the characters have children, friends, enemies, their actions set another plot going.
So you write, and rewrite, and carry a small notebook with you for ideas, and have one in the toilet, in the living room, by the bed, in the car, because ideas are and do come from everywhere.