My game would be bsed on Buster KEaton's movie "The General." In it, Buster has to retrieve his train (General) from the hands of the Union during the American Revolution. He chases after the train in another train that has a cannon attached to it. If you can ignore the fact that this technically would be a confederate win, my game would involve trying to stop the stolen train by shooting cannonballs at it.
Players would use the mouse (maybe touchscreen?) to calculate the arc of their shots, while using the arrow keys to have some control over their location so they can dodge the incoming canon-balls.
