I think that this is a very good question to ask after seeing the last episode.
We now know that Silas can create hallucinations that are intensely vivid, lucid, and seem completely real. What we don't know yet is the true extent of Silas's powers (at least of the "mind manipulation" kind). How far away can he use it, how long does it last, can he force extreme forms of hallucinations to affect (like Klaus fully believing the stake was in him) more than one person? We know more than one person can see him, but we don't know yet if he can create a hallucinated fire that affects a group to suffer from.
These are extremely important questions. Due to these questions it must be realized that from the moment Silas had a decent amount of blood available to him, anything we've seen or the characters have gone through could be elaborate "decorations" (i.e., when he drank all of Jeremy's blood).
He may already be using this ability on a large scale or already has used it. Also, due to the amount of blood he has taken from the hospitals, and if blood is the fuel source for both his abilities (or "magic") and immortality it may explain why he needs so much--but, it may also mean that he has something big in store for the gang. Is this magic or just an ability (like vampires having super speed, strenght, healing, etcetera--all powered by blood). Also, to what extent can it be used, can he affect dreams? I'd wager Silas has no access to expression magic, which uses emotions and sacrifices (though we still haven't had a great explanation of it). Blood doesn't seem to be related to expression, so it's either a new magic or his ability.
This is why he needs Bonnie more than likely, he can't use expression magic (considering that the spirits act as dictators over "normal" magic, expression must be used for a spell that goes against nature--immortality, ressurection, or taking down the barrier). Silas may also be using an extreme manipulation/hallucination to make Bonnie and her friends more pliable, to committ horrible acts all to get his objective met. Bonnie may also be his leverage against Qetsiyah when the barrier drops, she's Qetsiyah's blood and has her talisman as well.
The first obvious "super" mind manipulation that Silas may have pulled off to make Bonnie pliable and to also cause everyone that would normally unify against him, rally, and defend their home turf is incredibly plain as day to me: Jeremy's death.
Jeremy may be completely and utterly alive. Who knows where in the hell he is at though. Jeremy could've ressurected normally with his ring inside the cave and then found a way to climb out and start trying to get off the island (as I believe Rebekah would've been gone just in time to help Jeremy). If this is the case Jeremy will show up at the end to save Bonnie's some how and bring Elena back to her humanity.
It's fully possible that Shane is alive, but I doubt it. Silas needed blood and also needed to "take" his place (also he knew that he'd never be bringing back to life any non-supernatural being; atleast not yet).
This all hinges on whether that ring still worked on Jeremy or not. We aren't entirely sure if Jeremy was considered or was a hunter when Elena killed him (after she killed Conner and had the Hunter's curse). If he had the mark, something Damon and Stefan didn't know about till later (explaining Stefan's remark in the cave if Jeremy was a hunter when Elena stabbed him), then it's very possible the ring did work on Jeremy. Silas just has the ability to create a VERY powerful mass hallucination--and this could be the "something" Caroline was afraid of after seeing what happened to Klaus, the strongest of them all.
If Silas did do it, then atleast part of Jeremy coming back home (if at home then Silas took his corpse before the ring activated and put a hallucination in place; probably just before they smelled decay) was a complete fabrication. It's possible the entire thing was false and that Silas was Jeremy's dead corpse (or invisible) until they got back and he left and started again as Shane and went to Bonnie.
Anyway, I'll stop there and let everyone add to other ideas and also if you think Jeremy could be alive due to this new understanding of Silas's powers. For all we know he is powerful enough to literally make someone invisible (including himself--I'm sure he walks around like that all the time), Jeremy if alive could just be sitting, caged up, right in front of everyone.
It should also be noted that Silas's ability/magic IS very much like that of the hunter's curse. I think this is very interesting and possibly a hint by the writers. Also I've mentioned/written a link before that I think this further shows that Silas may have been the true origin for the spell that created "The Five"--created by a coven or witch that spilled blood in that cave, just like Shane. Possibly the same group or cultists Kol had talked about that worshipped Silas. Perhaps they had visions like Shane and that's why they followed him, also like Shane, they started a plan created by Silas to free him from bondage.
As a postscript, it could also be that vampires having the ability to compel may have a way to counteract this manipulation to some degree (compelling each other for instance, vampires compelling humans/werewolves/witches and the originals compelling the regulars to see things normally, if possible--also to stop believing or reacting to something unreal) or turning their humanity off or on may also free them (like Caroline's speech to Klaus creating an extreme emotional response which stopped it--turning everything off may help to never see it in the first place).
Anyway, if Elena goes to get Jeremy's ring and it's missing then we will know. We'll see what happens...