I think that it should because it would get the reader thinking and make them wonder what actually happened to her.
Well, actually there isn't that much of a mystery, either in the short original ending (which reads more like a part of a precise than a finished work) or in the expanded second ending. Both give an overview of Estella's experience in marriage. Since she is a character who is secondary to Pip, this is sufficient; we don't need to follow her through her experiences. Both give an indication of the directions Pip's and Estella's lives go after their final "interview," as the original calls their last meeting.
In the original, Estella believes the child, little Pip, to be Pip's son. He does not disillusion her on that point, which indicates he is content to go his own way and she hers. The reader can close the book knowing that if Pip ever marries, it will not be to Estella. In the second, I think kinder, ending, Estella is content to have seen Pip and parted friends; so we know that she has indeed given up her manipulative, cruel ways. Pip recognizes this and accompanies her into the evening mists that shed a gentle light on his future. The reader can close the book with a sigh knowing that there has been double redemption and that they might have happiness together.
So there isn't much mystery to the endings at all really. The reader has enough information through dialogue and symbolism (e.g., the evening mist's "broad expanse of tranquil light") to infer the future shape of Pip and Estella's lives--in one, separate live; in the other, joined lives.
I agree with the post which supported a level of the unknown. Authors do this for a reason...it makes us think, and we can, as readers, relate better to the characters this way. If it is spelled out for us, it is harder to put ourselves in that same boat if our lives are not so similar. This is why in many stories, the horrible creature is not fully described (we each imagine what is scary, therefore it is scary to all of us) or why the battle takes place off-stage (so we can each imagine for ourselves the atrocities of war and what would make it awful for each of us...not just a few). By leaving the end of the story regarding Pip and Estella to our imaginations, we are better able to relate, conjecture, and discuss in more detail exercising our creativity and our abilities to find textual support for those ideas. It also opens up the possibilities for a happy ending and a turn of events since they both have led relatively miserable lives.
The second ending which Dickens wrote in order to appease his Victorian audience mitigates the strength of his novel. Estella never really played a positive role in Pip's life, and with the resolution involving the return of the prodigal son to the warm confines of the forge and Joe, she has no place at this point in the novel and is better left out.
Are you refering to the ending of the story and the potential relationship between Pip and Estella, or the sadness she has experienced in her life after her marriage to "the Spider"? In a sense, I think it is good that both are left shrouded in uncertainty to some extent. As #2 points out, it is good to be able to impose our own understanding on these characters and imagine for ourselves what has happened to them. Also, in a sense, the subject of this novel is Pip. We know the precise nature of his own sufferings, so perhaps it is enough just to know that Estella has suffered equally.
I think that we should not know for sure what happens with Pip and Estella after the novel ends. I think that it is more interesting for us to be able to read the overall novel each in our own way and imagine for ourselves what sort of ending there will be. In that way, we can be free to create our own meaning out of Pip and Estella's relationship.