Kate is the mother of both Jacob and MIB by Bright_Star

Let me start off by saying that Kate MUST be important. She wouldn't have made it this far if she wasn't.

Anyway...on to the theory. I believe that Kate will somehow be in the position to raise both Jacob and MIB. Jack, Sawyer, Kate and the Kwons were almost pushed into the situations that led them to their guilt / situations. Jacob tell Kate (when she's a child) 'that she won't take / steal again' knowing she will take Aaron. So we are all thinking that he's hinting at her taking Aaron.......it is.....but it's more than that.

1) Kate kills her father, but tries taking care of her mother at the same time. 1/2 Black and 1/2 White.
2) She loves Jack the doctor. She loves Sawyer the conman. 1/2 Black and 1/2 White.
3) She loves the cop. She loves the robber. 1/2 Black and 1/2 White.
4) She escapes from the marshall but she saves him on the plane.
1/2 Black and 1/2 White.
5) She takes Aaron to raise him. She takes little Ben to the Others to save him. 1/2 Black and 1/2 White.

She's not a candidate but she's made it this far into the story.
Her story has always had ties to pregnancy / motherhood. She was worried she was pregnant with Kevin the cop. She helps Claire give birth. The Others think she may be pregnant after she's with Sawyer. Sawyer and Kate get into an argument after he tells her he's relieved she's not pregnant.

Maybe she was Claire's counterpart on the Agira flight of a pregnant woman after sleeping with Jack???

However, I think the most important point is 5. She would raise both a good child but take care of someone she KNOWS will cause pain and suffering because she can't hurt a child.

How does she get back to at least the 1800s to have both Jacob / MIB?........I don't know. But one thing is certain......Kate has always been 1/2 Black and 1/2 White.........so who would be better than Kate to raise both a "good" child and a "bad" child?

If anyone has a similar theory.....sorry. I don't read all of the theories and try to stay spoiler free.