With MIB and Mother being painted as Adam and Eve, that means Jacob can only represent one of two things (God or Devil, Good or Evil). I think Jacob represents evil. When adult Jacob was with Mother in front of the bright light, he said that Mother always wanted MIB to be the one who defended it. She responded by saying that now she understood it was Jacob's purpose all along. I think she lied, she knew that MIB was the only one who could truly protect the light, so she had to keep him on the island. In the scene before her death, it seemed pretty clear that she knew she was about to die and it's not much of a leap to assume that she also knew how Jacob would respond. By becoming one with the light, MIB was forced to protect it.
MIB is the actual defender of the islands power/secrets, but he's an unwilling defender.
Jacob plays a support role in defending the island by making sure his brother can't escape.
The candidate then is either the one who can continue to contain the MIB, or the person who can replace MIB. To replace MIB, the person would have to willingly embrace the island. John Locke originally fulfilled this role (so it's no coincidence that MIB takes his form) but Ben, an agent of Jacob, killed him. Now, Jack seems to be filling the role.
If I had to take a wild guess, I'd bet that Desmond becomes the new Jacob and Jack become the new MIB.
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