Benedict Arnold is definitely the traitor you are thinking of. He was an American officer who served almost directly under Washington and was quite celebrated at one time. Then he became more sympathetic to the British, causing his fellow officers to persecute him a bit. He in turn decided to turn traitor. He sold the British a key victory in return for land and power when they took back full control of the colonies. Instead, his plan was foiled and and he wound up serving in the British army and receiving a very small sum of money. After the war her returned to England.
As for who else to put on the money, the idea of british monarchy is good, though probably if the British had won, we just would still be using their money. The other possibility is to use different fouding fathers. Perhaps things were just slightly different at the time of the revelution and different people made it on the money - Philip Livingston, Roger Sherman, John Adams, John Jay, James Madison etc.
Another obvious turning point in American History is the civil war. Replacing Abraham Lincoln on the five dollar bill with anyone else would be pretty astonishing.
Hope that helps. Good luck.
Annie
