Aaron's not gone. Before coming back to the island, Kate gave him to Claire's mother to take care of. Kate's plan was to come back to find Claire and bring her home so she could raise him.
In fact, the whole mess last night with Claire looking for Aaron and saying she'd kill Kate if she were raising him reminded me of so much of that storyline from earlier in the series. Like how the psychic told Claire that she needed to raise Aaron. It seems that is beginning to come to fruition.
If I remember correctly (have not looked this up, this is just from memory) we learned in flashbacks that Jack's wife had been pregnant and... had a miscarriage, I think? Apparently in the flash-sideways, she didn't lose the baby, and David was born. He and his wife still divorced though.
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Who is coming? I'm thinking Widmore. I keep going back to him week after week! But he's been conspicuously absent this season, and he's the one who was always, always, always trying to find his way back to the island. He almost made it when he sent the freighter, and Ben rebelled against Jacob's orders during that whole time so it seems like Ben may have been going against Jacob's plans there.
And then, Widmore "changed the rules" when he killed Alex, which Ben swore he'd avenge, all the while Widmore is swearing he'll get back to the island. In the meantime, Jacob and the MIB have this whole discussion about loopholes, and some weird little ghost kid tells MIB he couldn't "kill him" because it was against the rules.
Rules discussions seem to revolve around Widmore and Ben. Attempts to get to the island seem to revolve around Widmore. I think he's the one trying to get to the island and Jacob is helping him.
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I want to know how Claire sees the MIB/smokey, since she clearly doesn't see him as Locke!