Meta-ish Speculation and free-association based on events and things said in 4.15.
We seriously got a lot of stuff in 4.15 that could lead into the rest of the season. Particularly from Alastair. Also, there's the death of Pam and some problems some people are finding associated with it.
First, Pam.
When Pam was stabbed, she didn't bleed. There was no blood on the knife. Bleeding didn't happen because the Reapers are out of commission, yes. However, this also explains why she asked Sam not to call a doctor, and why there was no attempt at medical assistance.
Reapers not being there means that mortal conditions or injuries are on hiatus. Aside from the hole made, completely stopped until the Reaper's return. The Reaper not being there wouldn't have any effect on non-lethal wounds. People are still skinning knees, cutting themselves shaving, and getting quasi-concussions from being thrown into headstones. Like Dean. Because there's no blood or anything more than a clean hole when Pam is stabbed, she would know that meant it was a lethal wound. She's a character who is very honest with herself, so she doesn't let Sam call for medical assistance because she knows it's pointless. That's my take on that.
Now. Why Pam died.... She was one of the very few active and recurring characters still alive and ostensibly on the Winchesters' side of things, and who knew what was happening. She was a resource that could be used, and could have been useful in future plot-lines. But it's pretty much down to her and Bobby for mortal Winchester allies, and both the strategic plot internal to the adversary characters of the show and the plot relating to the external season and series arc of the show needed the stakes to get ramped up. This episode did that in a big way on multiple levels, and I'll get to some of those in a bit.
Pam's death hammers a point home, that it's not a game, and it's not a safe thing they are doing, and that people who help them are in the line of fire. There could have been more i's dotted and t's crossed regarding the details of events surrounding her death, but the death or otherwise permanent irrevocable loss of an ally was a plot element that was necessary at this point.
And quite frankly I'd rather it be her than Bobby.
Moving on.
Things Alastair Said:
"Hard to get it up when you're not wearing your meat."
From this statement, it seems Sam's powers are directly connected to his physical body, specifically the demon blood in his body. Therefore, the "Sam is being trained to be Lucifer's vessel" theory gains massive likelihood. This then makes Ruby the schemer I've always hoped she was (training Sam up to be betrayed, and in doing so, alienating him from his brother) and not working for Lilith.
Lilith tried to destroy Sam with the nukey-hand thing. It didn't work, but she thought it would. I would guess from this she either intends to bring her own vessel to the game or wants Lucifer in a non-customized vessel (or not fully prepared vessel of Sam, since she's breaking seals at a frantic rate) so she can kill him and usurp his power. She's the one opening the seals, she's the one trying to get him out. Alastair is working with her, or at least is also working to break the seals, possibly with his own secondary agenda.
From this standpoint, YED's Psychic-kid seeding program would have been to generate the best possible candidate to lead the army of Hell on Earth (and handily setting up a power-structure and chains of command in the process, ready for when Lucifer rose and took over), break the seals and raise Lucifer (or remain ignorant as agents within the demonic power structure worked to do this), and provide a body that would be the best possible host for Big L. YED's reward for this... hm. I'm not sure, since YED knew Dean was going to kill him, and even brought the loaded weapon that would do it, I suspect there will be a return of Azazel. My personal theory of "YED is re-growing inside Sam" still works with his body being used for a vessel. The growing YED moves into Sam as a baby, fixes up the place and then moves out so Big L can move in.
Where does this leave Sam? If the demons' plans go as intended, royally screwed. And currently without the depth of support he needs to have someone see what's going on beyond the surface, get him to admit to himself what's going on and break the cycle before it's too late. If it gets to the stage where YED re-pops, Big L takes over, and Sam gets smushed down inside his own body, Dean's role in all this becomes clearer. He'd then be the only one who can reach the remnants of Sam and get him to fight back and not give up, while at the same time, trying to thwart what Big L is doing while running around in Sam's body.
(On a side note, I think we'll see a repeat of the 'knife in the chest' we got with YED and Grampa Campbell, only with Sam and either YED again, or Big L as the stabber. (Partly because I still have a half-written scene in my notebook, written during the season one hiatus, with that exact scenario.) Now that it's been foreshadowed, it's more likely.)
Enough babbling about that. Next.
"He doesn't really ride a pale horse, but he does have three amigos"
*flaaaaaaaaaaaail* Heh, sorry, reflex. The Four Horsemen of the Apocalypse. Death, War, Famine and Pestilence. Bring it, Kripke!!!
I've already said in an earlier flailing post, I suspect if Pestilence is involved, the Croatoan Virus will make a re-appearance in a big way.
I would so love if this happens. The Croatoan Virus has been a gun on the wall for far too long. It resurfacing in a big way, connected to the apocalypse, at this stage? Would be fantastic.
In the episode Croatoan then, Sam's physical body would have been being tested for resistance to the virus, as Lucifer's future vessel would need to withstand the plagues unleashed by Pestilence. This fits very neatly with the 'Sam as the vessel' postulate, and puts YED firmly in Lucifer's camp. Meg may have swapped sides between the end of Season one and Born Under a Bad Sign, since I believe Meg!Sam said something about her father's plans not being hers.
There's more stuff, this was a plot-seed and mytharc dense episode, and I'm sure better minds than mine are jumping all over it, but I'm on the verge of descending into random blithering and flailing, so I'm going to call that enough for one post.
Suffice it to say I want the entire rest of the series now.
(PLEASE, NO SPOILERS OR REFERENCES TO SPOILERY MATERIAL IN COMMENTS! The definition of spoiler for this journal is located in left hand sidebar. Theory and speculation based on aired episodes only.)