Walt and Aaron Will Become the New Jacob and MIB by Jeremy

I theorize that Walt and Aaron will become the two most important characters on lost. It works out perfectly as they're the the least popular characters on the show. A perfect sleight of hand by the show's creators. When Jacob said "They're coming," he obviously meant them. They have to come back to the island just like everyone else had to, just not at the same time. They are the culmination of the numbers. They are 108. Most likely all the canidates will have died fighting one another, setting off a chain of events that will allow for a plane or ship with Walt and Aaron in it to crash on the island.

I can see the finale now: An unnamed 30ish year old white guy and a 40ish year old black guy (don't forget time moves differently on the island) are seen on a plane/ship conversing. Meanwhile the show flashes to an epic battle between team faux-Locke and team Jack. Just as the last two combatants (most likely Jack and Sawyer) are about to engage or have killed each other, a giant plane/ship crashes with Walt and Aaron aboard. Later that night an unscathed Richard decides to have a long talk with either Aaron or Walt... Oh, and the alternate timeline folks regain their memories and live happily ever after.