"She is my future wife."
Translation:Lei è la mia futura moglie.
The article is only dropped before mio/a close relative when there's no adjective; either futura or promessa makes it necessary. If Manzoni drops it somewhere in his text, a modern Italian would correct him.
The published title is I Promessi Sposi. It's usual translation is "The Betrothed". While most future wives are indeed fiancées and so promesse / betrothed, English usage also allows "my future wife" to describe the speaker's intention to marry instead of a formal betrothal. So Duo's answer is a more accurate translation than yours.
A classic storyline is that a man says "I just met my future wife" as she leaves the room, and his life suddenly feels incomplete.