"The ending" can by definition comprise only the resolution, which is the bit when Darcy returns to Longbourn with Bingley in tow to reunite him with Jane. Austen has constructed the story so tightly that there is very little variation that is possible within the integrity of the world she has created, but two or three possibilities do come to mind involving Mrs. Bennet, Mr. Bennet and/or Lady de Bourgh.
Regarding the first, (1) Mrs. Bennet might be so true to her selfish and proud nature (and I never agree with the caricature representations of her in films, which make her into a farcical Dickensian character instead of an authentic Austenian one) that she could throw Darcy out of the house, then Elizabeth and Darcy would have greater obstacles to be finally resolved by her father's intervention.
Or--(2) Mr. Bennet might be so overwhelmed by Darcy's humility and aid and so horror-stricken by Mrs. Bennet's past prejudice toward Darcy that he must decline permission to marry Elizabeth, a quandary finally overcome by Aunt and Uncle Gardiner.
Or (3) Lady de Bourgh might storm the citadel of Longbourn and completely overwhelm the whole family--except Mary--who then shows her latent worth by citing some homily and putting Mr. and Mrs. Bennet and Lady de Bourgh to shame, thus saving the day.
In all scenarios, I can see no ultimate end other than a happy wedding (skipped over with the briefest of remarks) and a blossoming life for Elizabeth and dear Darcy at Pemberley.
While I can't imagine the novel ending with anything less than the happy ending for Darcy and Elizabeth, a more realistic ending would be a play out of the catastrophic decision of Lydia's running away with Wickham. Lydia's foolishness jeopardizes the entire family reputation, and if Darcy wasn't the wealthy man he is and hadn't been able to essentially bribe Wickham into actually marrying Lydia, all of the girls prospects for marriage would have been in serious jeopardy. In modern day money, Darcy paid Wickham hundreds of thousands of dollars to get married. The purchase of a officer's position in the military would have been extremely costly, and Wickham wasn't otherwise motivated to do the right thing. He probably regarded Lydia as a foolish girl who was willing to run away to have a good time. A modern audience would understand Austen's word elope to mean to run away and get married, but Austen's audience would have only understood the word to mean "to run away." Wickham had already proven that he wasn't interested in marrying a woman of no financial means when he turned his attentions from Elizabeth to Miss King. He certainly isn't going to change his mind for the charms of Lydia.
If Darcy hadn't saved the Bennet family reputation, he certainly would not have been able to marry Elizabeth and the probably would have continued to caution Bingley from marrying into this family as well. He would have been a disgrace to their good names.
There could many different endings to Pride and Prejudice. At the end of the novel, Elizabeth becomes engaged to Mr. Darcy, her true love, despite their differences. They overcome adversity and their own pride and prejudice to be together. It's a happy ending.
I don't believe another ending would be appropriate for the novel because in Austen's own life, the love of her life could not be with her. She lost her happy ending. He moved to Ireland (her parents did not believe that he could support her sufficiently) and they were never able to marry. Because she did not get her happy ending, I believe she gave Elizabeth Bennett hers. Her protagonist is able to "get her man", unlike Austen, who was not.
Possibilities of alternate endings would be Elizabeth and Darcy NOT ending up together and their pride and prejudice getting in their way of true happiness. Jane and Mr. Bingley could also have not ended up together, either, in an alternate ending, for example.