Hah, nice misleading title. We didn't actually learn more about who the island candidate may be, although Jack is certainly earning the title these days, but it was about L.A. Locke being a candidate for an operation procedure with which Jack could make him walk again.
Bernard, Locke's oral surgeon, was on the plane, although we didn't see him earlier this season, so Jack finally starts to notice (also thanks to sister Claire) how many people were actually on O815.

Locke had a private pilot license and crashed with his dad, which turned the old man into a vegetable. Locke's punishing himself for it by not wanting to be an operation candidate.
There is no sign of connection to his island-self yet except for him mumbling "Push the button".
Christian left Claire a small jewel case that's playing music and has a mirror. Somehow it made me think of the lighthouse mirrors.

On the island things are moving quite fast now.
The plane is rigged with C4, which makes UnLocke go for the sub instead. He switches Jack's backpacks to get everyone into a deadly trap under water. Sayid, who oddly enough became more and more himself again lately, takes one for the team and blows up. Not before telling the team that Desmond is still in the well.
Sun and Jin drown, Frank probably too

I guess it is now safe to say that UnLocke wants our Losties dead, so cheering for his side is certainly over. I wonder what role Claire will play, now that she is the last one dragging along.
Jack is convinced: Locke can not kill us, he is not allowed.
Which makes sense, because Smokey easily could have taken out the whole group within a minute.
But now he is out to finish the job, however he plans to do that.
Water is not as big a problem for him as earlier assumed. He can just climb out from it.

I think it is possible that next week will show no progress on the island. The sneak peak of "Across the sea" I tried to link below would give a hint, why. Sadly it has been removed as I was typing this. Which would also explain that the episode after that, "What they died for", refers to the tragic losses of Sayid, Sun and Jin.
At least on the island reality.