The subplot that would add tension to the story is:
one in which the character’s decision has dramatically different effects on a dear friend in each reality.
This subplot introduces stakes and emotional weight, as the character grapples not only with their own life choices but also with how those choices profoundly impact someone they care about. This can create a sense of urgency and pressure, enhancing the character's internal conflict when they confront the decision they must make at the end of the story.