I remember it being on AO3, and I think it was mostly that Steve would leave Bucky at the tower while on missions and Bucky ended up stepping in or something when someone tried something with Pepper, so they basically made a deal that he would get dressed up and taken along to a couple of functions. Pepper (may have) had Extremis still. I specifically remember a scene in which they were at an event, a bomb went off and I think there was something about Pepper's shoes breaking or something because Bucky was worried about the glass in her feet. There was another scene where he was re-arranging her hotel room too, I think.
It was not Five Reactions to Pepper's New PA or re: I'm keeping him. (though this one was very similar). Any help would be appreciated, thank you!