Doomsday Time Stone Theory
"You never once used your greatest weapon" Thanos brings up a good point
In the beginning of Infinity War Dr Strange tells Tony Stark twice that he intends to use the time stone. First in the Sanctum Sanctorum after Wong explains the infinity stones, "And this stone may be the best chance we have against Thanos." Second when they step outside the Sanctum, "You might want to put that time stone in your back pocket, Doc" "Might want to use it". But after Dr Strange looks into 14,00,605 futures he hides it and never uses it?
In the comics during the Time Runs Out issues leading up to Secret Wars, Captain America is kidnapped by the time stone and brought to various futures. He gets to meet a much older version of Franklin Richards and is able to ask him anything he wants about the incursions and end of the multiverse but his main concern is how to stop Tony Stark. The time stone eventually leads him to the end of time with the Council of Kangs.
My theory is that Dr Strange didn't use the Time Stone in his fight against Thanos on Titan because he sent it in the future in order to aid Captain America in his journey in Avengers Doomsday. Maybe this could be used to explain why RDJ is Doom, and maybe the time stone will eventually lead him to Loki at the end of time.
^(Bonus theory: Maybe Steve sees Sam sacrifice his life so bravely (similar to him jumping on the grenade) that it made him choose Sam as his successor over Bucky. As Dr.Erskine said, "Not a perfect (winter) soldier, but a good man.")