Ever since the release of Avengers: Infinity War last year, fans have pondered what, exactly, Doctor Strange's long term plans are after willingly surrendering the Time Stone to Thanos after the battle on Titan. Many have assumed the Sorcerer Supreme is playing a long game, one that extends beyond his death at the snap of the fully powered Infinity Gauntlet after viewing over 14 million possible futures (14,000,605, to be precise) before his fight, though it is unclear how this will ultimately unfold in Avengers: Endgame. However, a new fan theory may explain the rationale behind Strange's decision to relinquish the Infinity Stone he had sworn his life to protect in exchange for Tony Stark's life despite previously vowing to let the Armored Avenger die if given the choice.
Redditor Thomascgalvin suggests that, while viewing the alternate futures, Strange came to the realization that Thanos' use of the Infinity Gauntlet's full power was a one-time only feat. As seen following the iconic snap, the Gauntlet was damaged, possibly beyond repair, and having removed Etri's hands following the Gauntlet's construction, it's unlikely that a replacement can be forged. With that in mind, Strange was counting on Thanos using the fully-powered Gauntlet to render it useless following the snap before posthumously enacting his master plan to somehow undo its effects.
The biggest hole in this theory is that the Time Stone itself could be powerful enough on its own to reverse the damage to the Gauntlet or Etri's hands, as seen by Thanos using the Stone to restore Vision after his temporary destruction at the hands of Scarlet Witch. However, as the 2016 Doctor Strange film suggested, tampering with time comes with its own costs, including the distinct possibility of Thanos being trapped in a time loop if he potentially attempted to repair the Gauntlet. The Mad Titan could avoid this risk entirely, or it could spell out his final fate as part of Strange's larger plan.
And what of the titular "endgame" itself? The fourth Avengers film takes its title from one of the final lines in Infinity War, as Strange reassures Stark that they've moved into the "endgame" following Thanos departing for Earth to obtain the final Infinity Stone. This term, of course, comes with its own strategic implications, from a popular tactic in chess.
In the world of chess, an "endgame" strategy refers to the tactic of promoting a pawn into a more powerful piece and turn the tide of the game. Given Ant-Man's survival and confirmed role in Endgame, perhaps Strange is making an oblique chess reference to the size-swapping superhero taking on a more prominent role in the events of the film. The Quantum Realm has repeatedly been referred to as playing an important role in Endgame, and no surviving character knows the microscopic dimension better than Scott Lang.