1) Alt-universe Borg. Say, from the Mirror Universe. They reach "our" universe, generate a new Queen once they realize they're disconnected from the Collective, and start assimilating to rebuild their numbers.What makes you think the Borg are ever going to return in the main continuity of TrekLit? I don't see how -- Mack didn't leave any doors open for it. The nearly all-powerful Caeliar ended the Borg, period -- and he's said that part of the reason he did that was that he felt it was time to end the Borg's story.Surely just adding “and then the Borg returned” at the end of one of the STO “path” updates would have caused less confusion? A Borg return is pretty much inevitable anyway in TrekLit, sooner or later (hopefully later since most of us are still recovering from the Borg Overload).
2) Pre-Destiny Borg trapped in a) subspace bubble, b) black hole event horizon, c) Slaver stasis field d) outside conventional space-time, any of which would allow some quantity of Borg to escape the Caeliar ascension.
3) Experiments with reverse-engineered Borg tech goes horribly wrong and a new Collective is accidentally created. (Just because the Caeliar destroyed the Borg's technology, it does not follow that the Caeliar destroyed the knowledge of the Borg's technology.)
Sci, I agree with you, believe it or not; for the Borg, narratively, Destiny represents a final end. (Imagine Patrick Troughton saying that.

Just as the Daleks have escaped their final end (in "Evil of the Daleks," in "Remembrance of the Daleks," in the Time War and beyond), so too will the Borg. It's inevitable.