Here is my final end all theory...the odds are Im wrong but what the hell...here we go
ALT Jack 'fixes" Locke and after Des "shows" them the life they should be living they somehow flash to the MTL in physical form....
Jack wakes up in the jungle...runs to the beach and finds 815 crashed on the beach....
Locke wakes up on the beach...no longer paralyzed....
Jack and Locke died on Oceanic 815....
ATL Jack and Locke have to "sacrifice" the ATL lives they live....
Locke sacrifices Helen...Jack scarifices David...to become the new Jacob and MIB...
This all takes place in 2004 and explains how we get to the ending....
on the island in 2007 Jack defeats Locke/MIB and by doing so releases all the souls/whispers that are trapped on the island...
Leading to a teary good bye between Jack and all our dead losties...jack gets to say good bye to his father and finally Locke, who tells Jack "Thank you for believing me."
Finally, Jack sits on the beach...eating some fish...Locke walks up sitting next him....they speak...Locke turns to Jack..."Do you know haw badly I want to kill you?"...Jack turns back..."Yes."
screen goes black....L O S T