Originally, Kate was written as an older woman traveling with her husband, whom she is separated from after he goes to the bathroom right before the crash. This idea was still employed, but with the characters of Rose and Bernard.
The first twist would have been having a strong willed woman and a doting husband dynamic in the beach camp. Would Kate's husband tried to change his personality to become more forceful? Would he object to Kate taking a leadership role? Would this have been the Jack and Locke dynamic?
The second aspect of this would have been whether the survivors would have accepted Kate as their leader. Certain cultures, such as Sayid's, would have objected to a woman in charge. Other men could have put on a macho behavior in order to make themselves seem more important (for self centered reasons such as corrupting limited resources).
The third aspect of this would have been altering the Sawyer love interest. If Kate had a husband on the island, she would not be available to use her "charms" on either Sawyer or Jack. If she did, it would cause more conflict in the beach camp because her betrayal of her husband would have negative consequences with the other camp members.
If Kate and her husband were to be the Rose and Bernard characters, it would have been hard to mesh a story to fit their personalities. Rose had accepted her fate. She would have rather be left alone than make decisions or lead missions. Bernard only wished to make Rose happy. They found that mutual happiness by going off on their own into the jungle as introverts.
There were a few aspects of Kate's actual characterization which could have confused the plot lines. Kate was involved in many car crashes in her flashbacks. Car crashes were symbolic for change or even the after life. Kate was one of two characters to have escaped the island twice (Frank being the other person, who was key in getting them rescued.) Either the island did not have a great pull on her, or she knew something the other survivors did not know about their situation.
One could say that Kate's time on the island was a means of replaying her life relationships. The island characters were substitutes for her failed relationships. If she was in a purgatory state, she would have had to resolve these relationship issues in order to move on with her life or after life.
Off the island, Kate was a runaway, a criminal, a siren who manipulated men to help her. She learned that her cuteness was a powerful tool to get her own way. Her childhood friend, Tom, was a perfect example of this character trait. She led him on, then caused him pain. We can see the same threads with her relationship with Sawyer. When Kate finally thought she could change and settle down, she married a policeman and started to live the suburban wife lifestyle. But she never resolved her problems, which put a strain on that relationship. When things were getting close, she panicked and fled. We can see the same story line in Kate's relationship with Jack.
Kate had a streak of sentimentality. She courted a gang of bank robbers to create a situation where she could go into a safety deposit box to get Tom's toy airplane. After using the gang leader, Jason, she discarded him because he had little value to her anymore. Kate's relationship with Locke was at the same level. Kate would use Locke to counterbalance her disagreements with Jack or Sawyer. He was a buffer to get what she wanted; and the end she also discarded Locke who died as a result. Her one true girlfriend in the series, who she confided in, was Cassidy (who happens to be Sawyer's baby mama). She was Kate's one person support group when things got dicey. On the island, Kate tried to be that supportive role to Claire, under the guise that if Kate could save Claire (rescue her and reunite her with Aaron) then Kate could redeem and save herself. But this was really a substitution for her failed relationship with her mother, for whom she tried to "save" by blowing up the house with her abusive father inside. If Kate had an island friend, it could be said it was Hurley because Hurley was everyone's pal. He would listen to her, and not judge her.
It is possible that Kate so badly burned the closest people in her life that she could never see them again in the after life. She got her friend Tom killed in a police chase. She destroyed her relationship with her mother, who turned Kate into the authorities. She killed her bank robber boyfriend in cold blood. And she abandoned her husband, Kevin. That is why those people were not part of Kate's "awakening" in the sideways after life.
So in order to get past the sideways purgatory, Kate needed to find substitute relationships that would last the passage into the after life. The island gave her the opportunity to amend her past faults, be less selfish, and to make new stronger bonds with people. This may be why many viewers did not like the Kate character. There was always a hidden motivation that they picked up on the screen. Kate's motivation was to bond with other lost souls in order to move forward.
A few people never trusted the Kate character. Their suspicions fall from the fact that Kate was not seen on the beach where the middle section of the plane was found (including Marshal Mars). She was found wandering the jungle, unhurt. There are a few explanations for this: one, she was in a daze and wandered off the beach, but that is unlikely; or two, she was already dead but in spiritual denial. Her eagerness to dive head first into any mission was based in part because she subconsciously knew it really did not matter. Her outward strength would forge new friendships with the other lost souls.
Just as her time on the island and O6 story arc was conflicted and unbelievable at times, Kate's awakening was also written haphazardly. A mental light bulb was turned on and everything was great. She couldn't have been "waiting for a long time" for Jack unless her (after life) wait began when she first saw Jack near the beach after the crash.
Kate's cold detachment could have been the result of her character going through this underworld passage before . . . and failing. If car crashes are symbolic of death, then Kate had several "death" story lines to come to terms with her personal failings. How did she suddenly become an expert jungle tracker, a marksman, a mission specialist, and survivalist? Just from being an Iowa tom-boy?
It was probably a good decision not to have Kate as the survivors leader. It would have created too many tangent story lines. And her likeability factor waxed and waned during the series. But the idea of Kate being a focal point does give us a possible path into the deep roots of the series premise and themes if the goal of each character was to find substitutes for failed relationships in order to take charge with one's after life.
