If I remember correctly, I think Harry was enhanced by the goblin formula when the pumpkin bomb was thrown at him...Eddie wasn't....
As for bringing back Venom, what I think is that because the Spider-Man franchise has (at this point) such a great presence in the blockbusters universe that any new director that will be given a shot in this franchise might try to please the producers before trying to please the fans (I hope you get my point) and if this comes to be true, then we could easily see Venom again (and even Carnage), you can clearly see it even in the SP3 spoiler forum, one of the longest thread was the Venom one, Venom is a fundamental villain in the Spidey Universe (and a great milking cow by the way, which is what they might try to make in the movies if he ever reappears).
But I'm against a Carnage reappearance. First, I think it wouldn't be "fair" to give Carnage the screentime that Venom deserved better. Two, another symbiote would be redundant. Three, Carnage is just pure violence, and he would need a very very very great story to give him the substance he lacked in the comics.
Anyway, giving the statu quo in the movie universe, a carnage appearance (if they go with the symbiote sample way to explain it) he more likely than a Venom reappearance (I respect you points, but don't you think that making that small piece of symbiote that seemed to survive after the explosion regenerate Eddie a little bit too forced ?)