Well, based on your AU idea, each incarnation of Hylia goes into a fertility heat. When it gets to OoT Zelda's turn, she's in disguise as Sheik.
This could come into play in two ways:>A: The disguise is merely crossdressing aesthetic, and when the heat comes on, it gets harder and harder for her to hide her identity as her figure fills out.>B. The disguise is Gerudo magic, actually turning Zelda into a boy. However, the magic of royal lineage overrides the disguise in the presense of Link. Thus, when they first meet the heats start up, and Sheik turns back into Zelda during them. Having never had to experience them before due to the Gerudo magic strength, she's torn between hyper femininity of her true form and masculinity.
Her Gerudo-taught skills and discipline give her an advantage when watching over/assisting Link, as she has the ability to avoid him (and hide herself masturbating just out of sight.)
Over the course of the story, Link would put together that Sheik is a girl, and their interactions would gain a romantic connotation once she confides in him parts of her "condition" minus her identity. Could juxapose them as friends when younger, despite their low level of interaction prior to Link timeskipped, as to payoff when Link realizes that Sheik is Zelda.
(And now I'm thinking of a scenario later in the story where Sheik and Link have to fight off enemies together outside of the Shadow or Spirit temples, and Sheik's heat leaves her open to get injured, and Link has to tend to her wounds and in the process, undress her. That would be a perfect opportunity for Link to discover her secret identity, and examine her in close detail. And a scene in the Ice Cavern where Sheik's nipples are visible in the cold.)
Either case, gives a chance for Delicious Brown Zelda.