ok so, i have no skill in ANY part of game design, so:
what is it? a game where you, a harpy, have to haul things from point A to point B, moonlander style, in your stomach. there would be various mechanics, such as an upgrade system, physics(large cargo loads would “shift” with movement, causing inertia), and customization(wing and hair color, clothing, ect).
one thing i think would be neat, is that you would start out pretty small physically, and through the various upgrade slowly become bigger visually. bigger wings for more flight power, bigger body(either fat or muscle) for more stamina, bigger stomach so you can hold more cargo(with would probably either be liquids, or solids), so over the course of the game would would essentially go from being a small pigeon sized harpy, to roughly an albatross of a bird.
and that’s roughly about all i can come up with. feel free to grab this idea if you want to do something with it