Those are all good picks. It also made me think of Captain Garth, an experienced starship Captain who was insane and could shapeshift. Imagine if he got off the prison planet.
There was the cloud from "Obsession." It could fly through space, enter ships, drain peoples' blood. It was about to reproduce into thousands according to Spock. If they hadn't stopped it right there it might have spread across the quadrant.
If Kirk hadn't killed Gary Mitchell before he completed his transformation he would have been unstoppable.
Man, there should have been a Star Trek "What If" series at Marvel.
Ah Gary Mitchell.....I think there was a TNG series of novels that dealt with the barrier at the galaxy edge and it turned out that the Q created it to keep something out.
There was also the book Q squared that revealed Gary was channeling Q's power
Forgot about that cloud that drained blood, that was indeed a good threat and so was Garth