Sicario 3 was confirmed in 2018, but not much has occurred in terms of development. However, while many fans remain eager to see where a third film could take Benicio del Toro's Alejandro, a hitman who had a grudge against Carlos Reyes cartel in Mexico, the Day of the Soldado sequel didn't go according to plan for him, with the CIA burning him and leaving him for dead. But these unfortunate circumstances create an interesting avenue for the third film to make him the hunted and have a key character return.
Alejandro joined up with Matt (Josh Brolin) and the CIA once again in Day of the Soldado to pit cartels against each other, kidnapping young Isabel to start a war. It was a way to get Alejandro closer to her dad and the Matamoros' boss, Carlos, who ordered the hit on his family in Denis Villeneuve's first movie. And the Americans benefited as well, as it helped them control the border war while taking out Mexicans they felt brought terrorists into the U.S.
However, when the terrorists turned out to be domestic, Matt was ordered to end the job and kill Isabel because she witnessed them murdering Mexicans in cold blood. Alejandro also had to be cleaned up, which destroyed the partnership. Matt's actions eventually led to Alejandro being taken out by coyotes who wanted to curry favor with Carlos' narcos-running people, but Alejandro managed to survive and found the boy who shot him, Miguel. Day of the Soldado then ended with him prepping Miguel as an apprentice, but a third movie would be better using Alejandro as prey, not a predator.
Once the CIA realizes he's still out there, it can bring back Emily Blunt's Kate to kill him, under the auspices they're correcting Matt's mistakes. They can even have Matt working under her now to ensure Alejandro gets taken care of because he's a liability that can cause a political war. This would add tension because he and Matt were close and bring an extra complicated dimension to Alejandro and Miguel's journey to finish off Carlos' people.
This war on two fronts would be the ultimate test for a hitman, and seeing as Kate hates Alejandro for making her compromise her code, it's added incentive to come after him to ensure America's homeland security system is protected. It would also be a much-needed breath of fresh air for Alejandro, giving Miguel a trial by fire to earn his mantle as his successor.
Such a rivalry reignites nuance and torment in all the friendships the franchise has, and with the CIA's resources, it would cause Alejandro to step up his game against a familiar enemy. But more than anything, seeing Alejandro tracked and betrayed by the same people who turned him loose would feel like dark yet fitting karma for a series that prides itself on revenge.