Oh hell, who am I kidding? They never figured it out even with Jeri Ryan's boobs to work with. The Kazon were stupid, but Species 8472 were pretty awesome. I actually liked them more than The Borg because they were original and different, and reminded me of that one species from Babylon 5 that I can't remember the name of, but they were all CGI too. Yeah Jeri's boobs pretty much saved the show.
They ditched the Kazon years before 7 of 9 even joined the show. Their last appearance was the 2 parter between S2-3, and She appeared in the opener of S4.
The show did improve dramatically when she came into the show, but that's because they recognized the weaker cast, brought in a great new character, focused more on the best characters out of the rest, and more importantly, it also coincided with the transition to full CG for all space effects, which freed them up to do a lot of cool scifi ideas that the earlier series couldn't have done. They also came up with the idea of the Doctor's mobile emitter halfway through season 3, which allowed them to use one of their best characters more freely in a wider variety of episodes.
A ton of my favourite Trek episodes are from those 7 of 9 era years of Voyager, and it's a lot more than just Jeri Ryan's ample rack. Not that I'm complaining about that of course, as it certainly didn't hurt, but she was one of the best characters regardless.
And while Threshold was bad, there are other equally bad episodes in Trek. TNG did a similarly stupid episode with the entire crew turning into animals all over the ship. Every series has its share of equally bad stinkers, except maybe DS9, which had a pretty high average.
And I'm not including Spock's Brain in that. That episode is just good fun.