TPTB have said that viewers have enough clues within the first five seasons to figure out the solution to LOST. There can only be two solutions to LOST.
First, a Sherlock Holmes style final staged unraveling and explanation of the show. All the characters will be gathered together by someone (ageless Alpert, perhaps), who will recount their lives, their lies, their back stabs, their plans, their dreams, their motives, their crimes, their issues and their redemptions. He would then relate those personal items into the mechanism we call the Island, the master plan, or the end game.
Second, a Dr. Watson as coroner moment, where all the characters are shown to have died well before the 815 flight. Throughout the show, there have been tiny, glossed over facts about the main characters lives which could be the sole reality in a complex afterlife test. If you believe that the characters are dead, it is possible that they were dead before the plane crashed on the island. (The plane crash as a metaphor for passing onto the next plane of existence.) For example:
Hurley could have died in the porch collapse;
Kate could have died in the house explosion she set;
Sayid could have died during the Gulf War;
Jack could have died in drug overdose or beating during his odd Thailand beach trip (which does not fit properly into career time line);
Locke could have died at birth;
Sawyer could have been killed by his father in a murder-suicide;
Charlie could have died the rocker drug OD;
Eko could have died during his drug wars;
Claire could have died in auto accident;
Ben could have died at birth in the woods;
Ana Lucia could have died after being shot as a police woman;
Rose could have died of cancer; and
Michael could have died as result of being hit by a car.
It appears more than coincidence that the LOST characters have had life-death moments well before the island is shown in their lives. The journey to the afterlife may take many forms of transport (think Defending Your Life). The journey through the afterlife may take many forms, many tests and many dangers (think Egyptian myth of good and bad underworld gods). Time and space may have no meaning in the realm of the dead; the flashbacks or flashforwards may be dreams or nightmares or tests to determine whether the present (the time for judgment) affects a character's soul. I could see the end result is the realization of the characters that the lives they thought they lived were merely illusions (the name of the boat in the marina).