There are real monsters and there are fantasy monsters. Here is a list of a few monsters I have thought about:

1.  Politician – You are an alien from another planet and have come to earth to prepare the planet for conquering.  To help with your plans of world domination you try to become president of the United States. However, because of your hideous natural form you have to take on a disguise.  You disguise yourself as a toupee and mind control a candidate who is running for office.  The candidate has access to plenty of resources and seems to gain popularity as he continues to say the most meanest ignorant things.
2. Colossal Creature – You are a creature who is very hungry and misunderstood. All you wanted was something to eat but these little pests keep bothering you and preventing you from getting food.
3. Final Boss – A nearby town has had enough of your shenanigans and seeks to put an end to your life. To survive you must defeat all challengers while continuing to grow your power.

I’m leaning towards #3 since I have an idea of how the game play would work. #1 would require a lot of dialog but could feature some unique game play. #2 Seems to be more of a comedy.