I always thought it was a mistake to make the Vulcans the heavies in Enterprise. The writers did it, I suspect, for the worst possible reason: rationalism is very threatening to a lot of people.
By that same token, you know one Star Trek race I suspect is either secretly evil and has potential to be a great bad guy race...or at the absolute least, is a concept with a real dark underside the series hasn't really explored yet?
Hear me out, here.
What was your first, visceral, initial reaction on hearing about the joined Trill race? Isn't the idea of surrendering a portion of your personality, memories and will to an immortal superintelligent parasite that lives inside of you, which needs you to get around, a little…well, creepy? We're told over and over by main characters like Dax and others about how being home to a symbiote is an honor over which there's a lot of competition, but isn't it entirely possible the initial, visceral reaction is correct?
Let me put it another way. Would you want to be a Trill host?
Trill society, as we discovered in the Deep Space Nine episode "Equilibrium," is fundamentally based on a lie, and a lie that as yet, hasn't been exposed. According to the Symbiosis Commission, only one in a thousand humanoid Trills is suitable for joining, and they're usually chosen from that race's overachievers and geniuses. Yet the truth, as discovered in "Equilibrium," is that over 50% of the humanoid Trill could work as hosts. In fact, in a pinch, when the Dax symbiote was near death, the eminently unsuitable Ezri, as the only humanoid Trill nearby, had to become joined after a 15 minute lecture by a non-Trill doctor.
What's more, Trill history suggests a furtive secretiveness that could suggest a malevolent purpose - or at the very least, they're not exactly totally honest with their allies. The Trill had contact with and knowledge of the Federation, yet the Federation had absolutely no idea the Trill were a joined race at all until Beverly Crusher had to save Odan, the first Trill we ever saw (that we know of) in "The Host."
Consider this: Dax was alive and kicking all throughout the history of the Federation, and in one funny moment she even implied she had sex with Doctor McCoy when he was a med school student. "Well, he certainly had hands like a surgeon…"
Curzon Dax, at one point, was even the Federation ambassador to the Klingons.
Yet, the fact the Trill had a unique symbiotic relationship was totally unknown to a genius medical researcher like Beverly Crusher, who had a history of working with aliens. Odan himself in "The Host" only revealed it in a moment of absolute desperation when he was about to die. He didn't tell the Enterprise crew about it, and kept the matter a secret even though being a Trill was the explanation for a life-threatening condition (he couldn't be transported). In short, Odan kept being a Trill a secret for some unknown reason.
All this doesn't mean the Trill are evil, it does mean though, that the Trill have not entirely been forthcoming and honest all the time.
It may be possible we have encountered the Trill before TNG's "The Host." There was one eerie episode that stood out from the entire first season of Star Trek: The Next Generation, where evil insectoid, superintelligent worms able to bond with humanoids tried to take over Starfleet Command in secret.
This episode was all the more shocking because its scary resolution implied a follow-up that just never happened. We never got an explanation for who the parasites were, their ultimate goals, and what happened to them.
(In real life, this is because the original concept for the Borg were as insectoid hive-minders. Because the Borg became cybernetic for budget reasons, this original intro was unfortunately orphaned and forgotten.)
One of my favorite explanations for the insectoid parasites was they were advanced scouts for the Dominion.
Is it possible the possession aliens from "Conspiracy" were actually a version of Trill symbiotes?
Granted, the Puppet Master aliens are very different from the Trill, but then again the Trill have subraces. For instance, some die when beamed up and down, whereas Jadzia never had a problem.
The similarities between the Trill and the Conspiracy-aliens are numerous enough even non-Canon novels have mentioned this.
By the way, I did my best to avoid mentioning some of the key divergences between "The Host" Trill and DS9 Trill, because it was obvious in the TNG episode they were still trying to iron out the kinks of what the Trill are, including something that might be the strongest case for my "Trill are evil" argument: in "The Host," the Odan symbiont is the only thing responsible for the personality of the final being.
(This cool/rare image, by the way, is an early makeup test of Terry Farrell in "The Host" type TNG Trill makeup.)
Bear in mind I'm not saying all Trill are evil infiltrators or that Jadzia Dax was a sleeper agent or anything like that. I am saying as a whole, there's something kind of malevolent about the concept of the Trill that would lend themselves to being natural bad guys, and there is also a feeling we've never gotten the whole story.
And look at it this way: a future project that uses the Trills in a big way as heavies would be the greatest gift ever given to convention cosplay girls, who love the heck out of the Trill spot body makeup.