John of Gaunt (1340 - 1399 ). The rich and powerful Plantagenet prince. His liaison with a commoner called Katherine Swynford produced four illegitimate children who were given the name Beaufort ( He married Katherine in 1396 and their children, by this time adults, were legitimised).Their son John was the Great-Great Grandfather of King Henry VIII of England.
Arthur Tudor, Prince of Wales. Henry VIII's brother. He was the firstborn, the heir to the throne, and had he lived, he would have become King- and the entire course of English history would have been different. There would have been no Anne Boleyn.