Jacob and MIB built the FDW, which made them magnetized demi-gods by Another Other

Nothing new, I don't think. Just wanted to pull my ideas together before the show gets to it.

Jacob and the Man in Black are brothers, brought to the island sometime in the past, just like everyone else who has ever been on the island.

Jacob and the MIB work together to build the Frozen Donkey Wheel. Much like turning the failsafe in the hatch changed Desmond forever, building the FDW changed Jacob and the MIB forever. They were normal people, but were exposed to the electromagnetic energy where the FDW is, and became the entities they are today.

During the electromagnetic event, Jacob and MIB were turned immortals with opposite properties, like the two poles of a magnet. Jacob is white, MIB is black (obviously); Jacob pulls people toward the island, MIB is repellent, hence him wanting to get away from the island. Jacob rules the realm of the living, and the future, and is mostly concerned with the choices people make to bring the future about from the present. He is sure that people can change, and that the patterns of the past can be broken. MIB rules the realm of the dead, as he can appear as them, and is concerned with memories of how things were. Whatever happened, happened.

Together, they create equilibrium, a harmony in the universe. Jacob is quite comfortable on the island, as his magnetic pull wants him to remain there. The MIB is literally repulsed by being on the island, as his entire magnetic nature is to go away from the island.

I don't know where it goes from there, just some thoughts.