Headcanon that Mary doesn’t get married again, but in a true Titanic fashion the entire series is being narrated by her to maybe her grandchildren when she is an old woman. The last scene is her death and her reunion with Matthew, where they se eeach other once again in a room that looks like the Downton dressing room. There, music is playing and Matthew and Mary looks at Matthew, who smiles.
“Matthew?” she asks, and he walks over to her slowly and embraces her.
“You never got married again,” he says softly, sounding both guilty and relieved.
“Well, it’s like you told me,” Mary responds, wrapping her arms around him, “I’ll be your Mary. Always.”