The main reasons I came up with this theory was from last weeks episode and a dream I had...that I was watching the last scenes of lost...pretty pathetic yes I know
anyway my thoughts are that the losties in the sideways timeline will ultimatley reach their destinys without having to come to the island. Locke will walk with the help of Jack, Jin and Sun will work out their marriage, Rose will be heald, Claire will be with Aaron, Sawyer will meet the real Sawyer and get revange, Hurley won't be cursed, ect.
For the the final scene it will be a montage of our losties boarding flight 815 from syndey to la (yes I think they will get their again somehow) the monatge will talk about second chances, fate, and choices, all that good stuff, while very nice and sad music plays.
Meanwhile in the homes across the world...many lost fans will be crying and clapping and thinking about how they are very happy they spent the last 6 years dedicated to the show.