Well shit with enemy AI already in place within the game, there could easily be a single player mode with the Predator fighting the cartel and guerrilla soldiers.
Highly unlikely but a man can dream can't he?
A similar game called Evolve has AI bots replacing absent human players and Illfonic participated in its development too. So there is a chance it will be implemented at some point through an update if it is not available at release date.
Also I believe that Illfonic wants players to experience the game with other players first because bots can be flawed in comparison and there is a lack of communication if you play as a member of the fireteam with the other members controlled by AI.
Not to mention AI of enemy soldiers is not so sophisticated.