The candidate is not one of the chosen Losties but all of them. Evidences:
-Their numbers sum 108.
-108 is the number selected on the Lighthouse.
-108 were the maximum minutes allowed to be able to release/contain the electromagnetic instability of the dark matter below the hatch.
-MIB wants all of them to leave together the island. He has put a lot of effort in gathering them.
-When most of them left the island, they felt broken and came back.
-Jacks knows all of them have to do something together and so knows James, inside of him.