When Jacob told MIB at the end of season 5 that "they're coming", I believe that what he was talking about was right under our noses the whole time...
It wasn't the Losties/Candidates...
It wasn't Widmore and his science team...
I believe that, in fact, it was all the ghosts trapped on the island that he was talking about. All the people that died due to Jacob and MIB's actions and who were unable to leave.
Season 6 will end (this won't, of course, be the final scene) with the ghosts carrying MIB's (and Jacob's) souls to heaven/hell. They will get their revenge for what happened to them.
This leaves the question of how the candidate(s) is(are) important. And I believe that I have an answer for that. The ghosts will use the candidate's body as a host before killing MIB. That candidate will be either Jack, Sawyer or Hurley.
The war is, indeed, coming... all the lost souls against the island, MIB and Jacob. And once the souls are released - the island will sink. The Losties - every single one of them, including the ones who died on the island - will be released and will continue their lives in the Sideways.