A modest proposal is a work of satire. As such, there are really two main ideas. The main idea as it would appear if we take the story at face value and the true point of the author.
The main idea of the actual story has to do with decreasing the overpopulation by selling babies as food. Swift suggests that the wealthy purchase the infants of the poor and serve them as a delicacy. He gives specific details which are designed to disgust and enrage the reader.