Does the wonderful thing represent an unrealistic fantasy?

You could argue it either way. The "wonderful thing" is probably Nora's reference to a change in the forces of the world - the forces that not only keep women subservient, but also reinforce the bourgeois pressures to conform to a certain type of life, where everyone has an agreed upon place, in 'a doll's house.' It's not just that Nora would have to have more power as a women - they all would have to free themselves from their conformist pressures. Has this happened in the world since Ibsen's time? If you think it has, then it's not unrealistic (though it probably still it for Torvald, but the message is bigger). If you think we continue to fall into predictable patterns that limit our freedom, then yeah, it's probably unrealistic.