Of course, nobody knows when William will become king of Great Britain. His grandmother Queen Elizabeth II is already 92 years old and his father, and heir to the British throne, Prince Charles (69), turns 70 this November. The former royal butler Grant Harrold expects Prince Harry (33) and Duchess Meghan (37) to play a bigger role once Prince William (36) is king. As Harrold revealed to the Daily Express, "The Duchess of Sussex will have an important role to develop as one day she will be sister in law to the king". 

As Harold knows, this means that the couple will have more duties to fulfil. The Duke and Duchess of Sussex are already preparing for their roles, and have stepped in for the Queen several times already. 
Catherine, Duchess of Cambridge, who will become Queen, will have an even more important role, says Grant Harrold. "Both royal Duchesses have important roles to fulfill. I imagine we will see more of the Duchess of Cambridge as she prepares for her future role as Queen." 

Duchess Meghan takes her role as as a member of the Royal Family very seriously. She gave up her acting career before getting engaged to Prince Harry in November 2017 stating, "I don't see it as giving anything up, I just see it as a change". 

As Prince Harry's wife she has alredy made various public appearances and is one of the patrons of the Royal Foundation. She has even accompanied the Queen on one of her royal appointments!