SUMMARY - Steve returns the stones to the correct point in time but gets stuck in the past.
Bucky watched as Steve stood up on the platform, the case containing the infinity stones in one hand with Mjolnir in the other. Bucky's heart was pounding in his chest at the thought that he would never see his Steve again. Bucky knew that Steve was going to stay in the past and live the life he had always wanted with Peggy. Bucky's feelings didn't matter because Steve would get his happily ever after. Bruce fired up the time machine and Steve disappeared.
"How long will it take until he is back?" Sam asked, Bucky silently wished that he had Sam ignorance towards Steve's true intent.
"It can be hours for him but it will be about 5 seconds for us" Bruce answered. It felt like years had past as Bruce counted down from five.
"Bringing him back now" Bruce said but Steve didn't appear just as Bucky predicted, Steve decided to stay with Peggy. Bucky looked down at the floor as Sam and Bruce started panicking. He didn't want them to watch as his heart shattered into pieces.
Steve fell to the ground as he landed in 2012, the fight between the avengers and Loki was still ranging on. There were only two stones left which he had to return, the mind and time stone. Steve wouldn't lie when he had been tempted to stay with Peggy, but he knew she had moved on from him, she was engaged to marry her husband and even though Steve wanted to experience a life with Peggy he knew he could not affect the time line. If Peggy's future Alzheimer's affected her in a way which would reveal that Steve was her husband to his past self then it could change the timeline and the events Steve, and the other avengers worked so hard to change would be for nothing.
It wasn't hard quickly putting Loki's sceptre back into hydra's hands much to Steve distaste. The last thing he wanted to do was assist hydra to become powerful after everything they did to Wanda, Pietro and Bucky. Steve quickly snuck back out of Stark tower and headed towards the New York Sanctum Sanctorum to return the final stone. Steve ensured that this stone was the last one to be returned just in case something happened to the timeline which Steve would need to correct.
"Mr Rogers, I have been expecting you" the ancient one spoke.
"I am here to return the time stone" Steve replied before opening the briefcase once more.
"I am grateful however; your work can't end here. You took the stones and changed the timeline. For meddling with the timeline, you must in tern protect it. It is your duty from this point forward to ensure that each stone travels down it's expected path" the ancient one spoke. Steve's eyes widened, Bruce never said anything about having to protect the timeline, but it made sense. To protect his future, he must protect the past.
"How will I do this?" Steve asked reluctantly.
"I will train you will the powers of the mystic arts and using the time stone its powers will be fused into your being just like Carol Danvers with the space stone" the ancient one answered. Steve closed his eyes he wanted to return home and live the rest of his life with his weird family, but he knew this had to be done. For them, Steve opened his eyes which were filled with determination.
"When do we start?" Steve asked.
"Right now" the ancient one replied. Steve nodded his head before walking into the sanctum. Towards his new life. A silent promise left in his place. He will return to his family even if it takes one hundred years, he will return.