Could many of the Losties actually be Jacob's children? Well, at least the ones he visited when they were children, that would mean Kate and James.

Time travelers could indeed have children in the past.

Could John be Darkman's son? Imagine, not only John but Jack also. That's why John always said that he couldn't do it alone, that he needed Jack to help him, to be with him (on the Island).

Imagine (I'm letting my spin tales again) that Aaron is actually the grandson of both Jacob and Darkman.