25 black and white - use only two colors.

  Things that would look neat in black & white:
    - a solar system
    - cool crosshatching effects
    - silhouetting

  So there's some tree silhouetted against the horizon, and you have
  to defend it against little silhouetted invaders that run to try and
  kill it. You could also do day and night pretty well. Things that
  work as "tree defense": 
    - lightning
    - birds of prey 
    - earthquake (kills everyone but hurts your tree)

23	Light & darkness

  Solar system?
  Competing forces of light and darkness?
  Placing lights to light something up?

  Ok, you need to shoot lights in order to 

22	1 button

  Something like lunar lander where you have to use your mouse to
  steer a moving object. Single button controls thrust?

  You could control a ship at sea? A ship orbiting a planet or sun?

21	gore, gib and blood

  Inside of blood vessels or a body somehow? "Fantastic Voyage" is
  reference material here.

  Clever allusion to "operation"?

21 remote control - user input doesn't control the game's "main
character(s)" directly but rather through some other means

  You are a ghost or spirit or demon and you have to jump 
  between people and control them. They can't die when you're 
  "inside" or you die too.

  You control a character who can only be lured with external stuff.
  I.e. carrot & stick? Put attractive & repulsive "pheremones" plus
  things that trigger combat?

  That would be pretty cool.

