and, in Beaches,
Barbara Hershey had viral cardio myopathy, for God's sake. Try leukemia or multiple sclerosis or just Google "terminal diseases." Complications related to the protagonist's pregnancy are also acceptable, and you get bonus points if she dies within a minute of safely delivering the baby and holding it in her arms. A word of warning though: steer away from ebola, leprosy, or anything else that will require bringing Rick Baker on to do makeup effects.
2. Flawed boyfriend
The object of your main character's desire shouldn't be perfect in every way. It is every woman's dream to meet a man, fall in love, and then change
him. Perhaps he's too uptight, like Richard Gere in Pretty Woman;
or a misogynist, like Richard Gere in Runaway Bride;
or obsessed with his work and harboring a secret desire to break out of his rut, like Richard Gere in Shall We Dance?
Of course, outside of the solitary flaw, for which the protagonist is the perfect counter, the romantic interest must be perfect, thus making your main character "complete" him.
3. 60's soul sing-along
At some point late in the first act or early in the second, the main character and the people she cares most about (friends or children) must hear a soulful 1960's song either on the radio or a jukebox, and sing it together-ideally into combs, and while dancing about in a way that is carefully choreographed to look completely random and spontaneous.
Explore the discographies of Marvin Gaye and Sam Cooke. Nothing helps three-dimensionalize a white, twenty-something, upper middle class woman like having her a) know all the lyrics to and b) be driven to uncontrollable dance by Motown.
4. Strained mother-daughter relationship
The roots of this are probably Jungian. Jung proposed an Electra Complex as counterpart to Freud's Oedipal Complex, explaining hostility by daughters toward mothers. Whatever the reasoning, the ultimate chick flick must have an underlying theme of Matronly Disapproval of the Protagonist and/or Her Life Choices.