The wife should have her own career, hobbies, or "independent journeys". For example, if she is a merchant, her success could unlock new items for the player at stores like RETAILER_A_NAME.
Design romantic subplots that evolve alongside the player's main journey. For example, a character like Morrigan in Dragon Age becomes more than just a lover; her personal growth and relationship with the player directly influence the game's world and ending.
Use a system where choices shift a relationship meter in both positive and negative directions, similar to the complex systems found in Dragon Age: Origins.
The most memorable romances are those that tie back into the main plot.