William's title
Whatever title William and Catherine are given will revert back to "the crown" when Princes Charles become King, as William and Kate will then automatically become Duke & Duchess of Cornwall at that point.
Titles change with circumstance for example
Prince Charles title at...
New Title for Prince William
If we are to believe that Prince William & Kate will eventually live at Harewood Park near Hereford, they could be created Duke & Duchess or Earl & Countess of Hereford as these titles are free.
As they met at St Andrews they may go for a Scottish title - Ross...