Thanos' Dark Plan with Gamora and Nebula

One little known Marvel Cinematic Universe fan theory suggests that Thanos adopted Gamora with the intention of sacrificing her for the Soul Stone. However, as he raised her, he genuinely grew to love her and couldn't bring himself to harm her. To resolve this conflict, Thanos adopted Nebula, planning for Gamora to sacrifice her sister instead. He manipulated their relationship by pitting them against each other in combat, ensuring Gamora would always be the victor. Nebula's constant losses and cybernetic enhancements served to keep her at a disadvantage and foster resentment, making her more willing to fight to the death even if Gamora refused.

This theory explains why Nebula seemed aware of the price of the Soul Stone while Gamora did not. In both "Infinity War" and "Endgame," her comments suggest she understood that the sacrifice of a loved one was required for the Soul Stone's acquisition. The theory also speculates that Thanos might have had similar plans in place for other members of his Black Order, such as Proxima Midnight and Corvus Glaive, if Gamora and Nebula failed in their mission.

The idea here is that Thanos orchestrated a complex and emotionally manipulative plan to ensure he obtained the Soul Stone at any cost.