Answer:
Most Greek myths are about heroes and people who are or made their way to godhood. You could make up your own god/goddess, or a hero, or even a new monster. Most stories in Greek mythology don't make any sense whatsoever, so you could have a cat that breathes fire or something, and it would still make sense to all those Greek writers. I guess if its allowed you could make you own story about an already existing hero or god/goddess or monster. Maybe they are in love with someone, but they can't admit their love for them because they are already married or something. Maybe there is an unlikely hero who breaks stereotypes. It's your story, so use your imagination!
I hoped this helped!
Stay safe! <3