Remember the two skeletons in the cave, way back in season 1? Theories about who those two are run all over the place. Desmond and Penny, Sun and Jin, Kate and Jack, Kate and Sawyer, Sawyer and Juliet, Bernard and Rose.
One that doesn’t get much mention: Helen and John Locke. That’s who I have my money on. If John ends up surviving somehow (remember, “dead is dead”), wouldn’t he do anything possible to “fix” things in his life? If the Oceanic 815 flight never crashes, and they all get the second chance to change things, wouldn’t John try and patch things up with Helen? I think he would. Once he did that, I think it’s entirely possible that he’ll try and get her onto that island.
If John ends up being the one that saves everything (and as of this writing, I think he wil be) on the island, and ends up getting blown back into the past because of it, he’ll want Helen by his side.