Not that this is going to get much traction, but it needs to be recognize that the single most troublesome thing about the Borg -- their ability to adapt -- is a rather inexplicable addition to their capabilities that appears to be a misunderstanding of Guinan's line from "Q, Who." According to Guinan, "the ability to learn and adapt" was actually STARFLEET's main advantage, not the Borg.
The BORG'S main advantage is that you can't really
kill them. As Q put it "If you damage them, the essence of what they are remains. They regenerate and keep coming. They are relentless." And even in the early stages of "Best of Both Worlds" discussions about what to do with the Borg centered around THAT aspect, the fact that a Borg Cube could still fight effectively even if 70% of it was inoperable.
My sense is that this is and should always have been the main aspect of the Borg. It's not their ability to adapt that's the problem, it's the T-1000-like ability to get blown to smithereens and then pull themselves back together like nothing happened. If you want to make them more menacing, give them the ability to regenerate their entire ship in seconds or minutes instead of hours.