just making a disclaimer right now, if you do not, i repeat if you do not like my opinions or blogs, simply dont read this but really what im about to say isnt even opinion based, its fact based on the shows lore. 

but i got to thinking how cool it would be if like okay lets say we couldnt meet the shedim, maybe we could meet someone else who was teased but never introduced and i thought of the other knights of hell cain slaughtered but then i realized they probably couldnt show up either due to the fact of how they can be killed

basically due to certain beings weaknesses, it makes it harder for them to come back, lets look at each big bad and see if they qualify to come back

yellow eyes-he could return i think but with the colt gone, he'd be much harder to kill

lilith-i guess she could return but that would mean sam would have to drink demon blood again

lucifer-even as powerful as he is, he could still be stopped 

eve-like unless there is more phoenix ash somewhere okay but i was under the impression phoenix are rare to find, thus why they had to go back in time

the leviathans-they would just only be able to chop their heads off and put them in boxes cause i believe with the alpha vampire and crowley dead they couldnt make that bone weapon

abaddon-without the mark of cain and first blade, she'd probably be unstoppable or near it at least, i cant even say amara could merge with dean to make the mark cause that wouldnt make sense cause the mark was used to trap her inside so it wouldnt make sense she could get out anytime she wanted

cain-again same thing as abaddon, although you know we never actually saw dean kill cain truth be told, i always found that a bit odd

the alpha vampire-again without that colt, im not so sure he could die

amara-hopefully they wouldnt make her the villian yet again, so i dont even want to consider options with her

not counting the british men of letters sorry

the knights of hell-like azazeal, they could probably be stopped as jack was able to kill one of them, gabriel killed asmodeous but he isnt here right now, i forget how the other one died, crowley may have killed him

death and the horsemen-again, i dont even want to consider options if they are brought back 

the pagans-without certain weapons, it would be much harder to stop them but i think they'd be easier than some of the other creatures i listed, the pagans arent as powerful as this show depicts them to be

michael-i doubt he would be villian again, what just for jack to kill him again?, plus they got that archangel blade i think

not counting jack either sorry

also i wanted to make a point clear that because chuck is only trying to destroy the winchesters, he is going to probably bring back their enemies and not allies but im not saying that is impossible he would do that, maybe he would want to mix things up a bit but cmon, how would that make logical sense?, but there you have it, sadly because of certain ways to kill these monsters, it makes it harder for them to be brought back as enemies.

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