FOUND!!! Thank you to ellid for the pointer! And thank you to entropy_house for the addition details including the fact that It was not Steve Rogers from the Ultimates but rather EMH (Earth's Mightiest Heroes).
It is "Wonders of the Multiverse by Elenduen.
Story description from AO3: "Following Ultron Tony is alone rebuilding the Tower when Earths Mightiest Heroes Steve Rogers appears in his lab with a baby in his arms. His World has been destroyed by Thanos, Reed Richards got him and the baby out at the last minute so he and the baby could live and hopefully warn another world of the potential danger.
Tony invites EMH Steve to stay with his daughter, something that doesn't please MCU Steve, but he has other worries, as Tony is going through the Data Dumped files and comes across the Winter Soldier's hit of his parents. Cutting contact with the Avengers, Tony grows closer to EMH Steve and together they begin to build a team to face Thanos, and The Avengers when they come to an inevitable confrontation with Tony."