My idea for this BGMC, with the theme sci-fi, is a space game, where you are a small spaceship and and have to escort a larger spaceship, the Liberty, back to Earth (I am not sure if this is the definite location, might be somewhere else instead of Earth).
There will be other ships coming, acting like drones most of them that smash into you or the Liberty, but once in a while there might be a boss ship that can launch missiles by itself. In both cases you will have to shoot the missiles down.
Your mission is simple: PROTECT THE LIBERTY AT ALL COSTS
I am not sure if I will be able to complete the game, but I hope I will manage to get something playable. I might also work on the story, make it a bit more exciting. Already a few thing s come to mind…