You know it’s funny. We can think up theories of how this portent will come out, how Sam could end up being possessed by Lucifer (or something similar) by late 2014. But what I keep forgetting is - Sam isn’t even in this scene. This entire conversation is just Dean and Lucifer talking. So when Lucifer says “we will always end up here” - that we could mean “you, me, and Sam.” But initially, Lucifer says “you will always end up here” - you, addressing Dean and excluding Sam.
Let’s move to another interesting point. We just had an episode about Cain and Abel. Remember the last time the show mentioned Cain and Abel?
Michael referenced Cain and Abel when he was talking about how, although Dean is his true vessel, he could actually possess anyone in the Winchester bloodline, including Dean, Adam…and Sam, if he wished to. And if that’s true, then we must assume that Lucifer could, if he were in a pinch, possess Sam, or Adam…or even Dean.
But that’s silly, you say. Dean would never knowingly let a piece of Lucifer into himself! Except…
So there’s a small possibility that, in late 2014, we may still get this conversation between Dean and Lucifer. Only this time, the conversation may be a little more…internal.