And I think that when it comes to the Borg, people don't want the 15-foot tall lizard monster; they want the shadow in the closet, the creak in the night. They want the mindless force of nature, the monster that wants to eat you -- not the sociopathic megalomaniac with bad skin that the Borg became in the form of the Queen.
Personally, I think a mindless force of nature can be a great adversary for one story, maybe even two, but not on a continuing basis. What the Borg originally were, they can never quite become again, because as you say, it's the unknown that's truly frightening, and you can't unlearn knowledge.
So the key is not to go backward and try to recapture the essence of "Q Who" or "The Best of Both Worlds." The key is to find something new to say about them, or about the consequences of their existence.