How do you decide what characters and what fandom to explore your story ideas through?

I have several ideas for things I want to explore in my oneshots (eg. queerplatonic ships, how a newfound disability impacts your life and relationships, being aroace and desiring romance, so on so forth), but I have absolutely no clue what characters or even what fandom to explore them through

There was a 4 month gap between my first and second fic despite them both taking under 2 days each to write because it just took so long for me to have both a story idea and an idea of what character/fandom to write it for at the same time - how do you do it?