That’s the best lost promo this season. There is a clip of Desmond in this promo, and it looks like Sayid is trying to shoot him.
The centricity of this episode seems to be a secret. I’m not sure who it would be. The last episode ended with Locke being hit and the on island story is supposed to be about Jack and Locke coming together. So I think it would make since that the off island storyline would follow a story in which Locke and Jack meet at the hospital. Locke will have seen the other world and he will try to convince the man of science that this other world exists. I think this would be fitting because it could parallel the on island story and the whole Jack vs. Locke debate.
It might not be a Locke or Jack episode because they have already had their episodes this season. I would like to see a Claire episode sometime soon, she hasn’t had one yet, it seems like mst of her story was told in the Kate episode though.