I came up with this as a possible bloodline for the alt-Royal Family, starting with OTL's King William IV taking the American throne. In his place, Princess Charlotte survives and reigns into the 1880s, with her descendants on the throne today.
Monarchs of the Commonwealth of America.
1785-1837: William I (1765-1837)
1837-1842: William II (1794-1842)
1842-1889: Mary I (1800-1889)
1889-1919: William III (1862-1919)
1919-1946: George I (1870-1946)
1946-1992: Mary II (1892-1992)
1992-Present: George II (1926-Present)
Notes:
William I: Born the son of King George III, he installed as the Commonwealth's first monarch in 1785. A constitutionalist with reformist tendencies, the first American King is revered and respected in historical memory.
William II: The son of King William I, who is best remembered for carrying on the Hanoveran tradition of feuding fathers and sons. The King, upon ascending to the throne, began to assert himself politically and openly favorited Henry Clay during this time. Dies childless after a brief five year reign, and was widely unpopular.
Mary I: The younger sister of William II, Mary reigned for nearly fifty years and was widely popular for reinventing the monarchy and solidifying the concept of the constitutional monarchy. Tragically, her beloved husband died before the birth of their only child, Prince George, who would likewise predecease his mother in 1888. Dying of heartbreak after the loss of her only son and husband a year later, she was succeeded by her grandson Prince William.
William III: The playboy King, whose reign oversaw great industrial and military advancements for the Commonwealth. His last years were plagued by the First Great War, and in 1919 he fell to the Spanish Flu like millions of his subjects. Though popular for most of his reign, his lavish lifestyle had begun to grate on the public during his final years.
George I: The younger brother of William III, King George I was a more introverted and prudent monarch. He presided over the Great Depression and led the nation first through the Pacific War and then through the carnage of Second Great War. Dying just weeks after "V-Day" (Victory Day) from lung cancer, he was succeeded by his daughter Mary.
Mary II: The longest lived monarch to date, Mary reigned for 46 years, nearly beating the record of her great-grandmother. The Queen's reign saw the Second Pacific War and the ensuing introduction of the atomic age and the Cold War. Dying in 1992, she was succeeded by her eldest son Prince George.