Scrolling shoot 'em up
Example game idea: a side-scrolling or top-scrolling shoot 'em up game, along the lines of DonPachi or Tyrian. Use 2D accelerated rendering, highly optimized to allow many moving, animated objects (bullets, ships, etc.) on the screen at the same time. Support joystick, mouse, and keyboard controls. Support local same-screen cooperative multiplayer. Use SDL_mixer for sound effects and music.