Kate will never be Queen lol.
You may get King William in the future as you hope, but his wife doesn't become Queen though. The PROPER title of king or queen can only be passed on through heredity, not marriage.
When Prince William does become king, Middleton would become the Queen Consort – “Consort” being the official title of the reigning monarch’s spouse – and would be known as Queen Catherine, though she would not be line to rule if anything happened to her husband. Instead, the throne would pass directly to one of her children, likely the oldest male, or if offspring too young, to Prince Harry, William’s younger brother.
Normally upon marriage, royal men receive a title as a gift from the current monarch. For William, he’s become a Duke. Upon his Dukedom, William’s wife has become a Duchess (of Cambridge), if William hadn't have accepted the title, the couple’s titles would simply be as Prince and Princess William.
If Prince Charles takes the throne as king, William would be bumped up to the title of Prince of Wales, making Middleton the Princess of Wales, a title that now belongs to Camilla Bowles, who has never used it because of the sensitivity to her involvement in the divorce of Prince Charles and Princess Diana. Instead, she uses the title Duchess of Cornwall, which would also pass to Middleton if Prince Charles were to become king. It’s widely hinted that Camilla would then take the title of Princess Consort, instead of Queen, to avoid public scrutiny. Still, Charles may choose to pass on the throne entirely, and let it pass directly to William if the Queen’s current reign carries on for many more years – as it could.
