What do you think Helmer means with his final line in the play A Doll's House?the quote is "the most wonderful thing of all"

I think this is a great question, especially since the
play, whichever version one reads, is a translation.  And there are many translations of
this play out there.  For example, my translation, which is by Rolf Fjelde, gives
Torvald's final line as:

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Nora!  Nora!  Empty.  She's gone. 
The greatest miracle --
?

Whichever
phrases is used "the most wonderful thing of all" or "the greatest miracle," a key
observation about this line, in ascribing meaning to it, is that he is repeating part of
a previous line of Nora's, part of an exchange that comes just previous to this final
line:

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Torvald

Nora
-- can I never be more than a stranger to
you?

Nora

Ah. 
Torvald -- it would take the greatest miracle of all
--

Torvald

Tell
me the greatest
miracle!

Nora

You
and I both would have to transform ourselves tothe point that -- Oh, Torvald, I've
stopped believing in
miracle.

Torvald

But
I'll believe.  Tell me!  Transform ourselves to the point that
--?

Nora

That
our living together could be a true
marriage.

So, it is, first,
of importance that he is repeating a phrase, an idea that was introduced by Nora, almost
as if it were a foreign language that he cannot understand...yet.  It is also
significant that he is quite focused on this idea, since the last image that the
audience has of the play is not of Nora leaving, but of Torvald considering this idea. 
This final image suggests to me that his consideration of this idea is, potentially, as
important a transformation in the play's as the one that Nora has already
undergone.

The second thing that I notice about my
translation of this phrase is that it ends in a dash (which, in a dramatic text,
signifies that the speaker is interrupted, either by their own thoughts or by another
speaker) and a question mark.  So, this suggests to me that he stops himself from going
past these few words in summarizing what Nora has said must happen for them to get back
together, because he does not yet understand what it means (hence, the question
mark).

So, Ibsen has very pointedly left Torvald alone
onstage to have the final moment of the play, rather than Nora, suggesting that the next
step, after Act Three is over, is in his hands and that he is fixated upon living up to
this "most wonderful thing of all," if he can figure out how.