The "miracle of miracles" is that Torvald Helmer would love his wife Nora as an equal—that is, he must recognize her as a person, not as a "doll wife."
Near the end of Act III of A Doll's House, the "wonderful thing" for which Nora hopes is that her husband will demonstrate an unselfish love for her after he understands her past actions. Such a love would be comparable to the love she has for him, and their living together "would become a true marriage." She hopes that when Torvald reads Krogstad's letter, a wonderful thing will happen: her husband will tell Krogstad, "Publish the thing to the whole world," and he will sacrifice his honor because he understands that Nora procured the loan to enable him to get well in a warmer climate. But this does not occur. Only when he receives the second letter from Krogstad, which promises that no action will be taken, does Torvald forgive her.
NORA: As soon as your fear was over--and it was not fear for what threatened me but for what might happen to you--when the whole thing was past, as far as you were concerned, it was exactly as if nothing at all had happened. Exactly as before, I was your little skylark, your doll....
Since nothing in their relationship has changed, Nora tells her husband that she is leaving, and she returns her wedding ring to him. When Torvald asks her if he cannot be more than a stranger to her, Nora replies,
NORA Ah, Torvald, the most wonderful thing of all would have to happen.... Both you and I would have to be so changed.... That our life together would be a real wedlock. Good-bye.
Nora has no chance for individuality in her marriage, so she parts from her husband and her children in order to discover herself and no longer be a "doll."