I find this sort of thing interesting so maybe you all do as well!
How to address
Kate Catherine Middleton after her marriage?
It’s tradition for the Queen to bestow a dukedom for a member of the royal family after marriage, and traditionally it’s a dukedom for an heir to the throne. (FYI the rank order is king/queen, duke/duchess, marquess/marchioness, earl/countess, viscount/viscountess, baron/baroness).
If Price William declines the gift of a dukedom, Kate will be known officially as Princess William. This is the title for a woman who marries into the royal family, but not royal by blood, if another title is not given (i.e. Princess Michael of Kent).
If they do receive a dukedom, there are only six dukedoms currently available in the U.K., meaning there are no incumbents or family members currently occupying them. (FYI a dukedom is given by the queen and passed down by family, but if an heir is no longer available the title reverts back to the Queen to re-distribute).
Albany – title was suspended in 1917 after the duke backed the wrong horse in WW1; duke also later joined the Nazi party; his great grandson is could now technically petition for restoration for the title so it is not truly vacant
Cambridge – Likely the winner!
Clarence – the first duke, noted in Shakespeare plays, was executed for treason during the War of Roses. It took 300 years for the title to be deemed fit again and that duke’s son was later enveloped in a scandal with a gay brothel and rumors that he might be the infamous Jack the Ripper.
Connaught – This dukedom is in Ireland so not ideal for a future King of England.
Cumberland – title was suspended in 1917 after the duke backed the wrong horse in WW1; the great grandson, Ernst August, husband of Princess Caroline of Monaco, is actually the heir to this dukedom and could petition for restoration for the title so it is not truly vacant
Windsor – This was the title create for King Edward VII when he abdicated to marry Wallis Simpson in the unforgettable scandal – leading to Queen Elizabeth to eventually ascend the throne.
Sussex – Potential winner, but may sound too close to the Early and Countess of Wessex and cause confusion.