Elizabeth Bennet made an unusual main heroine because she was so outspoken and “arch,” as Austen calls her, while demure Jane would never say or even think a critical word against anybody.
I have found another example of this sweet-girl/saucy-sidekick switcheroo, in the sprightly two-volume novel The Portrait (1783) by a “Miss Elliott.”
Maria Bellmont is the sauciest of the saucy. Naturally I assumed she was a saucy sidekick because The Portrait opens with a letter from her sister Charlotte, writing to their friend Harriot Marchmont. I thought Charlotte would be the main heroine, but Charlotte gets married in the first volume, and the lively Maria takes center stage for the rest of the novel.
Not only that, but the deus ex machina in the novel is another "girl power" female, Lady Mortimer, who comes up with a ploy to help Maria win the man she loves.