After rejecting my first two ideas AND the two following prototypes I decided I will try another idea, and for the Commodore 64 in assembly (thanks to @hamrathdoesldjam !) and finally I'm having fun!

It will most probably be a JAM entry because I can't imagine finding the time to do some original music and/or sound effects, but there are things happing on screen. And I'm only using 33% of my processor power (for a commodore 64 that's a third of 1Mhz). So I think I will end up with a game.

By the way: those dots on the screen are bullits, they're consuming most of my processing power now.