Through a roundabout way, Jack and Kate became the lead-romantic figures in the series. Jack was saved from being axed early on for dramatic effect (which ironically Boone would serve that role) while Kate was originally to be the lead lead character, but got downplayed to partial secondary lead (on the missions as surrogate for the fans).
Boone and Shannon could have leveraged their rich arrogance into strong willed aristocratic rule upon the beach survivors. Shannon was quite the manipulative rich girl who was used to getting her way. Her wiles were downplayed in the series to only be a doting, spoiled, lazy brat. Her character could have been written in a more bold fashion. She could have used her sex appeal to bond loyalty with various male characters, such as Sawyer, Sayid and even Locke. Once she got muscle behind her, she could set herself up in island luxury (almost in the same mold as Ben).
Boone was smart enough to run a large business for his mother, he should have had the skill set to "manage" the castaways in an efficient manner where they would look up to him for their survival, instead of Jack. In fact, Jack was still afraid of authority figures due to his father's belittlement of his skills. Boone could have been the skilled politician who cut the deals to keep everyone in line.
Such a cutthroat couple was not foreign in the writers, since they used that concept with the misguided introduction of Nikki and Paulo, who conspired to kill a wealthy man for his diamonds and wealth.
A ruthless Boone-Shannon couple, linked by their dark secret of a sibling affair, would have added a new layer of darkness to the series. It could be used to keep them together, or later, pull them a part.
