While Raul Castro looks healthy at 85, he has vowed to step down in February 2018 during the next communist party congress
The man in line for the job is Vice President Miguel Diaz-Canel, who is relatively young at 56, outside the small circle of guerrilla war veterans and not a family member.
The grey-haired Diaz-Canel, who favors jeans or more formal suits and ties, has been a discreet communist party operator who lacks Fidel Castro's charisma and Raul's military experience.
But he has been "a good soldier in the shadows," Christopher Sabatini, an international policy lecturer at New York's Columbia University, told AFP.
President Raul Castro named Diaz-Canel as his deputy in 2013.
While Raul Castro looks healthy at 85, he has vowed to step down in February 2018 during the next communist party congress, though he would likely remain the party's chief.
His brother, who was buried on Sunday after dying on November 25 at age 90, clung to power from 1959 until an illness forced him to hand the presidency to Raul in 2006.
Tall and affable, Diaz-Canel lacks the oratory skills of Fidel Castro but Raul gave him a ringing endorsement when he named him vice president in 2013.
"Comrade Diaz-Canel is neither a novice nor an improviser," Raul Castro said.
The electronics engineer is an advocate of a more critical press and an opening of the internet on the island, where a tiny fraction of the population has web access.
"Today with the development of social media ... and the internet, prohibiting something is almost an impossible pipe-dream. It doesn't make sense," the vice president once said.
Diaz-Canel, who is from the central province of Villa Clara, gradually worked his way up the echelons of Cuba's single party.
In 2003, he entered the influential 15-member politburo, a key step for any official with higher aspirations.
Six years later, Raul Castro named the former university professor as his higher education minister.
In March 2012, he became one of the eight vice presidents on the council of ministers.
He rose to the more powerful post of first vice president on the council of state a year later, replacing veteran revolution fighter Jose Ramon Machado Ventura, 86.
Since then, Diaz-Canel has often traveled abroad, either with Raul Castro or as the president's representative.
But he is not the only member of the younger generation to have risen in ranks in recent years.
Foreign Minister Bruno Rodriguez, 58, was a key player in the diplomatic rapprochement with the United States and the European Union.
In a sign of his growing influence, Rodriguez appeared with the Castro family, top military officers and Diaz-Canel outside the armed forces ministry in Havana last week to watch Fidel's ashes depart on a cross-country trip to his burial place in eastern Cuba on Sunday.
Rodriguez and Diaz-Canel were the only members of the new generation of communist officials at the symbolic farewell.
Another relatively young operator is Marino Murillo, 55, an economist who is in charge of the crucial but modest economic reforms that Raul Castro has implemented.
Raul's only son, Colonel Alejandro Castro Espin, 51, is also gaining influence in the interior ministry.
All of them are keeping a low profile. Others who appeared too ambitious in the past lost their jobs unceremoniously, such as former vice president Carlos Lage and former foreign minister Felipe Perez Roque.
For Ted Piccone, a senior fellow at the Washington-based Brookings Institution think thank, Diaz-Canel is the natural political heir apparent.
"I don't see any other rival. I don't see any other scenario, but we've also seen in the past younger leaders groomed for leadership and they've been eliminated or removed from power," he said.
To help his successor, Raul Castro needs to speed up the reforms to fix the troubled economy, Piccone said.
"The legitimacy of the post-Raul government will depend on a much better economic performance," he said.
Sabatini said Diaz-Canel faces "three competing pressures": The old guard in the politburo, the "younger generation of party apparatchiks who want change" and the military.
But Raul Castro would be watching over him, as he would not be completely retired as party chief.
"Raul's and Machado's support can prop him up" as president, said Arturo Lopez-Levy, a Cuba expert at the University of Texas.
"Once he is in this position, it will be up to him to navigate successfully the task of building a consensus without humiliating any of the factions that form the Cuban leadership," he said.