How can I write another ending, an alternate ending, of Emmaby Jane Austen? The story is already perfect and has a happy ending, but I have to write...

You
are correct when you suggest that you need to materially alter the plot
to develop a different ending. The way Austen constructs her novel,
there is no ending but the one she has written. There are one or more changes to the plot that
might be made that might lead to one or more alternate endings, arising as they will do from
alternate plots. Some characters that might undergo plot changes might be: Jane Fairfax, Harriet
Smith, and Mr. Knightley. Of course, plot changes to one of these characters would redound upon
and involve other characters.

Jane Fairfax:
Starting with Jane Fairfax, she might become permanently disillusioned with Frank Churchill and
abandon him and their secret engagement to actually take on governess duties for the Sucklings.
In this case, Frank would be free to be paired with , who might pity him, or with Harriet
Smith,. whom he might pity, especially after he comes into his fortune upon his aunt's death. As
a side not, secret engagements were so very bad because there was nothing to prevent other
eligible persons from paying court to either party and thus finding themselves dishonored and
possibly disgraced once the secret became known. The reason for this is that once a courtship
reached a certain consistency and state, an engagement followed by a marriage was expected in
order to preserve the moral purity of the parties courting.

Harriet Smith: Aside from the possibility mentioned above
stemming from a plot change to Jane's fortunes. Harriet might undergo her own plot change by
truly casting off Farmer Robert Martin and attaching her heart and hopes to Mr. Knightley. This
might end one of several ways. She might end alone after rejecting Martin in London and
unfruitfully attaching herself to Knightley. She might actually persuade Knightley or her
worthiness and love. In order for this latter one to occur, Knightley would have to undergo his
own plot change.

Mr. Knightley: Mr Knightley
might truly see Emma as a different sort of person than he had always imagined (or hoped) her to
be. The outing to Box Hill might very well bring about such a changed perception. He already
scolds her for her disrespectful ill-treatment of Miss Bates. He might see more in Emma's
insensitive behavior and flirtation with Frank and become deeply disgusted with and repulsed by
her so that his scolding was more than that, so that it was in fact a breach from her because of
her breach from that which is worthy and admirable. This might then drive Knightley to
appreciate and embrace the innocent, unspoilt simplicity of Harriet Smith. For Knightley, having
the high social position and prestige that he does, this connection could not demean him in
Highbury the way it would demean Elton, who is lower in status and prestige.

These are some possible plot changes the would yield alternate
endings. Another possibility for deriving an alternate ending is
to approach altering the plot through the lens of a particular literary
theory. For instance, you might rework the latter parts of the plot through the
lens of Feminist Criticism, where the women triumph and are independent from men, or through
Marxist Criticism, where wealth is bested by meager income (Jane, Harriet, Miss
Bates).

were [Miss Bates] prosperous, I could allow much
for the occasional prevalence of the ridiculous over the good. Were she a woman of fortune, ...
I would not quarrel with you for any liberties of manner. Were she your equal in situationbut,
Emma, consider how far this is from being the case. She is poor; she has sunk from the comforts
she was born to;...