MEXICO CITY  When Fidel Castro promised to step down this week as Cuba’s head of state, he left the Cuban people and the rest of the world wondering not only what role he will play in retirement, but what changes will follow in the country he has run for nearly 50 years.

“And what now?” said Annia, a 30-year-old student in Havana who declined to give her last name to a reporter from Agence France-Presse. “We’ll see what happens. We have to be ready. I just hope whatever comes, the situation will get sorted out.”

Most experts on Cuba, along with many Cubans, expect that Raúl Castro, the 76-year-old younger brother of Fidel Castro, will step into his shoes. Raúl Castro, the defense chief and first vice president, has been serving as acting president since mid-2006.

But it remains a mystery whether the elder Mr. Castro, who is 81, will split up the two most important jobs he holds as head of state  the president of the Council of State and president of the Council of Ministers  to give one to a younger loyalist. It is also unclear whom he will choose to replace his brother as first vice president.