After the unexpected death of Prince Charlotte of Wales in childbirth in 1817, the various younger sons of George III scrambled to produce an heir to the throne (William IV, Edward Duke of Kent, and Adolphus, Duke of Cambridge all married because of this, and Edward's daughter of course became Queen Victoria). The rare exception to this was Frederick, Duke of York, George III's second son, and therefore directly after George IV in the line of succession. Yes, in 1817 he was married, to Frederica of Prussia (whom he apparently hated, and never had a child with), but she died only three years later in 1820. What if Frederick is also motivated enough to remarry? Who might he marry, and assuming he has a child in 1821 (at 57 I think that's barely reasonable, as his brother Adolphus had a kid at 59 IOTL) who comes to the throne in 1830, and what could their upbringing and then reign have been like?