Richard doesn't really look like Jacob. Given he's Hispanic and Jacob seems pretty white to me. Even if Jacob's wife/lover was Hispanic I doubt any child would look like Richard. By the same token we can probably rule out: Michael & Sayid. I think we can also rule out Locke (It seems pretty definite who his dad was. His dad wouldn't have come after him for a kidney if he wasn't his actual son), We saw Ben's birth and his father's resentment to him, so I don't think it is Ben. Kate's Mom and "dad" both seem pretty sure that Kate killed her biological father, so that put Kate out. Hurley's parents seem pretty tight now, and loving, so I don't think Hurley would be Jacobs son either.
I don't think such a big reveal would be wasted on a brand new, supporting character like: Bram, Caesar or Illana, because, who would care?
I also don't think it will be Juliet, because A) she's on another show now and won't be on Lost for more then a guest appearance or two this season. B) We've seen why she was brought to the island, her scientific skills, not because of another reason.
So that leaves Jack, Sawyer & Desmond.
All could be good contenders. Jack would be the obvious choice since he is the main guy on the show, but his father seems pretty sure he's Jack's dad. And acts like it.
Could be Sawyer, but I don't know why they would pick him, for that role. It would seem out of left field if that were the case.
So that leaves Desmond. We've never seen/heard anything about Desmond's parents. He has flashes of the future, he jumps through time, he was "drawn" to the island. If Jacob and his adversary are battling for the island, with Ben and Widmore as "pawns" in the battle, well Desmond has ties to Widmore and Ben and their battle for the the island. It wouldn't surprise me if we found out that Widmore was the son of Jacob's adversary.
So I think Desmond is the most logical choice for child of Jacob. It would make the most sense thematically, structurally, etc...